Changing pitch

[CDK]

The bronze 14x17" props I purchased for my tunnel drive project seem too heavy a load for my engines.

Someone suggested I had the pitch changed by a company called "Federal".
So far I have been unable to locate such a company, but is it possible to substantially reduce the pitch of a prop or is it just a quick way to ruin them?

[TollyWally]

From what I understand you can go about 2 " of pitch. I'm going to have my spare prop repitched as an experiment. It's tweaked anyhow so it won't cost me much to experiment.

I'm sure we can expect some confirmation or denial from the assembled brain trust.

[Eric Sponberg]

The bronze 14x17" props I purchased for my tunnel drive project seem too heavy a load for my engines.

Someone suggested I had the pitch changed by a company called "Federal".
So far I have been unable to locate such a company, but is it possible to substantially reduce the pitch of a prop or is it just a quick way to ruin them?

CDK,

Here in the US, Federal Propellers is a brand name from Michigan Wheel Corporation. Here is their website:

http://www.miwheel.com/Propellers/InboardPropellers/Federal.aspx

Michigan Wheel could repitch your propellers for you, however, that seems a huge shipping expense to send them all the way over here. My suggestion is to try to locate a good propeller shop, perhaps in Italy. Any good propeller shop should be able to repitch your propeller for you. It is not an elegant process; usually it takes a good bit of hammering on the blades to bend them to a new pitch. As a result, there is a limit to how much pitch you can change, and 2" is about that limit. The alternative is to buy another set of propellers with the same diameter but different pitch. It might be worthwhile to consult with a naval architect or the propeller shop to see what they recommend for your boat. One that I can recommend here in the US is Platzer Marine Propulsion Inc. Their website is:

http://www.platzermarine.com/

I hope that helps.

Eric

[daiquiri]

CDK,
I've understood that you live in Croatia, so it would be much simpler for you to have the job done in Italy. I can recommend you a firm which have made several well-done prop re-pitch and re-size jobs for my clients. They are
"Fregonese Bruno e Figli" Officina Meccanica s.n.c.
and they are located at San Donà di Piave (VE) - near Venice.
Their phone number is
+39 0421 221080
The fax is:
+39 0421 222209
E-mail: fregonesesnc@libero.it

They have a vast on-field (or in-water? ;) )experience with props and will be able to evaluate the ammount of pitch change necessary to achieve your goals.

Hope it will help.

[Frosty]

First get a big bowl of cement and bed the prop into it. Before it dries pull the prop out , this will be a base from which you will know how far you have changed or if you want to go back.

Then carefully decide where it should be hit. Carefully decide on what you are going to hit it against and with what,-- a big ball pain would be best.

To be honest changing the pitch will be the leading edge and trailing edge. This will seem like a change in pitch.



The thing is if they are no good to you what do you loose. If it improves it almost to where you want it the props can be melted down and re cast.

[daiquiri]

Frosty, changing a prop pitch is a job which goes well beyond hitting it with a hammer or whatever.
A prop has to be balanced statically, dynamically and hydrodynamically, and it takes right tools to do that.
Unless you are a fan of hull vibrations and worn-out shaft bearings... ;)

[TollyWally]

Frosty,
If you are serious could you expand on that? I have seen some amazing things done with 3rd world technology before and seen some pretty impressive feats pulled off by old timers. While my first thoughts are similar to daiquiri's my mind is open to the possibility. I will admit I am a bit taken back by the thought of recasting a mistake but I get it. Do go on.

[gonzo]

You should consider if you have too much pitch or diameter. Propellers can be cut down

[Frosty]

I remember as a kid saving up for a new equipoise prop. It was a bit too pitchy so I took back this jewel like prop wrapped in a scratch proof towel.

They knocked the crap out of it with a hammer !!!!.

Since then I have altered pitch at the side of the river with a hammer myself.

I have talked to Sonni Levi who used to do the same thing.

Granted 14 inch is getting big. I don't know what the professionals use but I doubt if the hole in the hub could take the force required to re pitch a blade from its root, I think at best it would elongate the shaft hole.

Altering leading edge and trailing edge would get you a couple of inches and thats Max.

A car panel beater would understand how to hit the metal to make it respond the way it is required to.

Maybe I should'nt have started this.

Basically I don't know how they do it but just telling you how I do it.

The point being --if they are no good as they are now what do you loose.

[fasteddy106]

Cdk, I think you would be far better off selling your props on ebay or some similiar site and buying props with the correct pitch for your boat. Frosty, I shudder to think of what would happen to my 38 year old OMC Electric Shift if I used the method you just endorsed. One of the 40ft Vikings with twin diesels in our marina hit a bait bucket backing into his slip and the vibration from the bent prop made me want to take a leak every ten minutes when we went out the next day. Ended up diving and removing the prop and having it balanced. Cdk, if you are looking for just a bit more performance you would be better off having the correct props cupped. My boat originally came with a 14X14 prop. When I changed the engine to a custom built 205hp V6 that prop just made the engine rev way too high to get anywhere. I went to a 14X18 prop and now the boat tops out at 25mph @ 4000 rpm. That is an increase of 10mph over the old engine, a 145hp kaiser V6 and 5mph increase over the new engine with the old prop.

[Frosty]

No-No- No- you take the bloody things off the shaft first.

You have to very carefully decide where to hit them. It may take a long time to make something to hit them on , that is a very important part of it.

You know what ---leave them,--- buy some new ones geeeeez

[CDK]

Frosty, I do appreciate your attitude. If I ever find myself marooned on an uninhabited island I'd wish you were there. With a hammer of course.

Because no hammer in the world is big enough to change the angle at the base of the blades, the result of any pitch change is a compromise I guess. I do not know whether or not prop makers use a different cast for each type or set the pitch for 14x16, 14x17 and 14x18" using the same basic prop.
Would it be a more elegant solution to flatten the blades with an hydraulic press instead of a sledge hammer?

[Ad Hoc]

what power is going to the prop and at what rpm?

[Frosty]

No I did'nt say a sledge hammer and this is not third world get you out of the shit thing this is what they do. Thats what I said --you call it the base it is called the root, that can not be twisted but the leading edge and trailing edge can, -----theres a terrible echo in here.

I give up. Talk numbers and RPM then go buy new ones.

[tuantom]

Saw a show on making stainless steel props once - Mercury or maybe Michigan Wheel - and they really let the hammers swing on the props to adjust the pitch. Brand new. Takes a healthy shot to bend one up too.

[apex1]

Would it be a more elegant solution to flatten the blades with an hydraulic press instead of a sledge hammer?

That is the way it´s done in a professional manner. "Umdrücken" is the term in German, do´nt know how to translate that (English is´nt precise enough).
And the 2´max. range given above is right! The prop root cannot be manipulated. You are better off, I guess with a new pair of wheels.

Regards
Richard

[CDK]

what power is going to the prop and at what rpm?

Engines should yield 85 hp at 4600 rpm, torque approx 170 nm. The reduction is 2:1.
At full throttle there is 3000 rpm, so 1500 at the prop, which is not enough to get over the hump. This may sound way off target, but I think 500 turns more might do the trick.

[baeckmo]

Sorry, ones and zeroes are not my friends....

[baeckmo]

Ooops what happened here, how do I delete??

[baeckmo]

So, what kind of speed did you get with those props, then? And how much of a tunnel are we talking about, and what kind of boat (planing/displacement, weight, lwl et c.)?

Note that a propeller in a tunnel normally will work in a considerably retarded flow (high wake factor), so the output from "standard" calc procedures will often overestimate pitch.

On the art of heating and beating propellers; considering the labor the producers spend in making the blade profile perfect, within tenths of millimeters, a good propeller is easily spoiled by overeconomical stupidity and a hammer! Those props are eating from your fuel budget for a looooong time, and you easily loose ten percent or more if hitting the wrong spot!!!!

First, give us correct basic data as above. Then we may come up with what you should have had. And finally, sell/swap the ("unhammered") screws you have and get your hands on a correct pair.

It is easier to make a good increase in pitch (mostly done by sensible cupping) than a good reduction! So when ordering a prop for a new project, select pitch on the low side! Best twister I've seen is a guy here in Sweden (Richardson); he is using heavy clamping mechanism for hub and blade sections, allowing him to twist the complete blade to a new pitch, without changing blade profile; nice job! Of course, this generates a slight difference in pitch between root and tip sections, but it is far better than the s-secton resulting from terrorizing leading and trailing edges.

[apex1]

Ooops what happened here, how do I delete??

Hi baeckmo,
you can delete a post by using the edit function in the lower right corner.
And thanks for your valuable input. I am sure CDK will provide the link to his thread showing the arrangement he has built.
Obviously you know what you´re talking. I have noticed that already on the Jet related thread.

Regards
Richard

[Frosty]

"terrorizing leading egdes and trailing egdes"

That is all you can do the root can not be altered.

Pressing a blade into a preformed cast would be nice,--if you got that stuff, yet "tweaking" a blade for a small adjustment would not be necessary.

To adjust a blade to advance 1 inch more than it did does not mean the blade is bent 1 inch. Pitch is the average of the blade anyway and not all the blade would be the same pitch.

To advance by means of pitch a 1 foot dia prop the leading and trailing edge would be altered by a very small amount like less than 1/4 of an inch and would be altered as high into the blade as possible.

Depending on the blade only one of the edges may be altered if you have a very aggressive leading edge for instance and you wanted less pitch that would be sufficient to alter just that.

Some props are easier than other due to the thickness of the blades which when you get to 14 inch dia bronze it can be difficult.

No alteration of pitch is as good as the cast pitch but the idea of a 3 stepped blade by the altering of trailing and leading is not so, its a very slight alteration and if a concrete cast is made of the prop before hand as I have already explained the pitching procedure can be reasonably accurate.

[Ad Hoc]

CKd

"...Engines should yield 85 hp at 4600 rpm, torque approx 170 nm. The reduction is 2:1. At full throttle there is 3000 rpm, so 1500 at the prop, which is not enough to get over the hump..".

Ok, im getting a bit confused now.

You start off by saying you have a 14x17" prop
1) Is this selcted by you, or by the engine supplier or prop supplier?
2) What was the speed with a different prop and its size?
3) What is the expected speed of the boat and what is the actual max speed at max RMP?

Since you're now talking about getting over the 'hump'. This is a totally differnet issue. And must not be confused with you're orginal post of pitch/diameter of a prop.

So I need to be clear what is the "problem" first before wading in.

[pistnbroke]

I am with frosty ...we have mended many props here at the Noosa Prop Shop ..welding 2 inches on to the end of the blades of a bashed up 13 inch aluminium prop to restore its diameter ...grinding by eye and thinning with a few bangs on the anvil ....they all worked fine and some better than the fancy equivalents and never had any vibration problems ..mind you cast bronze is not aluminium or red hot stainless .....now that bends easy .....get the oxy out .....

[apex1]

Since you're now talking about getting over the 'hump'. This is a totally differnet issue. And must not be confused with you're orginal post of pitch/diameter of a prop.
So I need to be clear what is the "problem" first before wading in.

John he made a tunnel covering the prop, but has limited waterflow at the suction side making issues.

Let us have the link to your thread CDK.

Regards
Richard

[CDK]

John he made a tunnel covering the prop, but has limited waterflow at the suction side making issues.

Let us have the link to your thread CDK.

Regards
Richard

Here is where I first formulated my idea. It was so weird nobody responded:
http://www.boatdesign.net/forums/surface-drives/sterndrives-jets-surface-drives-18742.html

Then there were the details, written while I was constructing:
http://www.boatdesign.net/forums/surface-drives/diy-tunnel-drive-20433.html

The choice of props. I have studied general engineering, but "general" at the time did not include much hydrodynamics. Lots of aerodynamics, probably because one professor was fascinated by the subject. So I asked the forum for help:
http://www.boatdesign.net/forums/inboards/simple-propeller-question-19421.html

And then the project showed some flaws:
http://www.boatdesign.net/forums/boat-design/tunnel-drive-performance-issue-23715.html

I've skipped several other threads like the marinized VW diesels, the unorthodox drive train and the electronic throttle and clutch controls. These parts went exactly as I intended.

With bated breath,
Cornelis Koger

[Frosty]

Tunnel drive performance issues has a picture of CDK's tunnels.

Its hard to see really the length of the tunnels which at first glance don't seem to be long enough. I think your vibration and inability to get onto the plane is that the props are drying out due to the tunnels not allowing enough water into the tunnels. I would think that that is the vibration too where the prop blades are leaving the water and when the go back in they recover thrust so your thrust is not a constant flow but thrust in each blade. Surface props are prone to this hence 4 -5-6-blades.

You could try some surface props or get the ones you have cupped which then they may tolerate both situations.

In one of your threads above 632 days ago I did say you would have prop issues and would need to play with many props.

I would say that your tunnels should be about 3 times the dia of the props and the tunnels should have a gentle rounded edge to its entry to encourage water flow if not it will break free like it was the transom.

Please get some more pics of the tunnels ( if its out of the water)

[Ad Hoc]

CDK

briefly viewed your threads.

Well the basic problem, as far as i can see, is that you doing it back to front. You're now trying to design a boat, that has already been built.

The design procedure will have more than likely prevent much of what you have done, simply because any naval architect would have told you it wont work or if you do decide to continue XXX will happen or this way is better or will cost more etc. Which is not what you probably wanted to hear....(no one likes criticism on personal projects).

So you have a hump issue, that is more problematic than the pitch...Hump issues are major and fiddling about or focusing on the pitch is wrong. You need to address the fundamentals of the design first...and is the design achievable or not. If it wont work from the start, then no amount of pitch change will assist!

What waterline length is the vessel, how much does she displace fully loaded and max HP of engines. This will give some indication of the what the speed "should roughly" be.

[CDK]

Ad Hoc, Frosty, I can't blame you for not reading the whole roll of threads, but believe me, there are no problems with the amount of water entering the tunnels and there are no vibrations. I wrote about that at the end of the "tunnel drive issues" thread.

There is almost 2 ft of tunnel between the gently sloped entrance and the prop. Just because there is enough water there is no air pulled in from the sides. Instead of more than 4000 rpm and a lot of vibes at full throttle I can only reach 3000 now with 10 knots readout from the gps. The bow lifts between 1 and 2 ft, but this Norwegian design with 26 ft. waterline weighs 3,5 metric tons and there simply isn't enough torque to increase the prop rpm beyond that point.

I have means to increase the engine output well over 100 hp at near 5000 rpm but the torque at 3000 rpm is all the engine can give. My target however was a maximum speed of 22 knots, which was stable planing speed when the boat was still equipped with twin 140hp Mercs.

At the present I am quit happy coasting along at 6 or 7 knots with a fuel consumption of approx. one tenth (!) compared to the Mercs at that -or any other- speed. So I did really achieve something with this weird concept.But the sea in front of my door is a treacherous one that may turn mean even on a sunny day, so I want the possibility to quickly reach a sheltered bay.

[Ad Hoc]

Well, based on your rough figures above, your best speed possible is going to be around 16~17knots, max. If you have bought everything expecting 22 knots, that is where you problems have started. Not matching the propulsion train delivery of thrust to the design limit.

[Ad Hoc]

just looked in more detail at some of your pix....judging from what i can see in those few pix posted, i would say there is also a flow problem for the prop, which would reduce the max allowable speed, to somewhere around 13knots, maybe a tad more if youre lucky, or less if you're not.

So to get up over the hump, around 9knots for your boat, there may not even be enough power being delivered at best anyway.

[CDK]

just looked in more detail at some of your pix....judging from what i can see in those few pix posted, i would say there is also a flow problem for the prop, which would reduce the max allowable speed, to somewhere around 13knots, maybe a tad more if youre lucky, or less if you're not.

So to get up over the hump, around 9knots for your boat, there may not even be enough power being delivered at best anyway.

Please explain to me what flow problem you see, because I don't. But I'm eager to learn.
The lower half of the prop is below the boat's bottom, the upper half has almost 2 ft of tunnel to draw in water. The exit is in no way restricted, the rudders are positioned to the left and right. Between the tunnel roof and the tips of the prop there is more than 1" clearance.

With restricted water flow I expect a prop to cavitate and the rpm to increase. In the current setup, the opposite is the case.

[Ad Hoc]

CKD

You still need to address the power to weight ratio and hence the maximum top-end speed you could ever achieve. You need to accept that you wont much more than 16~17knots!, in an ideal world.

Im sure you’re familiar with the concept of the water being slowed down by friction from the bottom of the hull so a propeller is working in a wake, which is being dragged along with the hull? Hence a wide ranging prop efficiencies, of the same prop and installation, at various flows of inclination and obstructions to the prop. To the extent that a boat may doing 8knots yet the as far as the prop is concerned its only doing 6knots, so an estimate of the wake is required. There are rough guidelines, in so far as props in open water, ie below the hull has 0%, behind a skeg, but not below the hull, and slight curvature changes to hull, around 10% and so on, up to figures around 30% which translates into, poor “inlet” flow. It is this reduced water velocity that is used for prop calc's, not the boat velocity. Hence wake is important.

With a duct, the water flow is very dependent upon the flow in the duct, not just the boat and its shape iwo ducting. One of the important parts of duct is the shape of the lip on the fwd and aft edge edges. The ideal is not to cause separation of the flow, because you want as much boundary layer flow from the hull as possible. In other words to draw as much low velocity (not speeded up) boundary layer flow as possible from the hull fwd of the duct. If the shape and angle is too great, separation occurs and you get poor flow into the duct which seriously affects the wake and hence thrust from the prop. Endless studies of angles on flow into ducts have clearly shown that 25 degrees is the upper maximum. Which is why most waterjet manufacturers use around 22.5 degrees, to go as high as possible (so not making the duct too long and problematic) to ensure that there is no, or minimal flow separation and the consequential losses in the duct.

Your duct looks around 45 degrees, ie not ideal and hence all you’re doing is making your own private Jacuzzi!

[Frosty]

What he is saying is that the water coming down under the boat wants to go in a straight line as does everything. If the tunnel is too quick and short it wont turn the corner and go up into the tunnel. He quotes 22 degrees as the tightest corner it will go into but sooner or later speed will even stop that.

However you are right is saying that you would get cavitation or airiation under these conditions but are getting bogged down.

I assumed from your post your were getting jacuzi type white water and no power to get over the hump but if the RPM is staying down then we are back in pitch problems.

I don't like your 1 inch clearance,--at all,- infact if that is 1 inch as the blades enters the tunnel then that is your vibration.

I think the rule of thumb is 1/3 rd of the prop dia for normal under hull prop position and a bit more for rudder clearence.

By the way you can easily remove 2 of your rudders and gain better steering.

[Ad Hoc]

If the RMP is low, even under the aerated condition, it is a mechanical issue, not hydrodynamic one. Since the prop would ostensibly be turning in mixture of air/water, but as far as the prop is concerned it would be air, ie free to race.

There are many 'design' aspects that are not right Frosty. But one must deal with the important issues first, such as the lack of power to go beyond 16~17knots, and the flow into the duct/props. The rest are mere details..

[Frosty]

One of the problems Im having here is information. its seems to be spread over three threads.

How much Hp have you got, length ,weight , gear ratio.? Lets start again.

I have just noticed your diagram of your tunnel, that is not a tunnel, it should start under the gearbox at least. I know I know --you dont have clearance !!!! such is tunnels.

I think you have a huge hump because you have taken away some of you planing surface under the rear of the boat for the "tunnels" -not much but it would act like a negative trim tab. You are down on power and it cant make it. Having the engines moved back would have made it worse

Being at such an angle on the hump means water is getting into the tunnels and the props are just about living with it. I think if you did get over the hump the props would break free into surface mode which of course they wont live with.

I don't know what to suggest but start again, actually you are closer to a surface prop configuration than anything else,--fill in the tunnels and fit surface props!! But even that would be a compromise.

The only thing I can suggest is try some wieght forward and try to get it onto the plane.

Of course as Ad Hoc say you must have the power to do this.

[CDK]

I would have made much longer tunnels if it were possible. But the existing hull, designed for twin stern drives, did not offer more length.
This design with an oil filled stern tube and thrust bearing in the tube itself, needs to be flanged to the transom, so the tunnel location is dictated. The engine bay length proved to be just long enough for the engine, gearbox, universal joint and prop shaft coupler: there is less than 1 inch between the belt pulley and the bulkhead which separates the engine bay from the fuel tank.

With this setup, fuel consumption is reduced by almost 90 %, so I must have done something good as well. Some 20-30% may be contributed by the VW diesels versus 3.0LX Mercruisers, but the rest can only be the much higher efficiency of the propulsion.

The Mercs - with a cleaned hull - gave this boat 28 knots top speed at 4600 rpm and stable planing at 22 knots, 3500 rpm. Mercuiser claimed 140 hp, but these boats were also sold with twin 120 hp Volvo's which performed equally but needed some more time obtain speed.
Instead of 240 (Volvo) or 280 (Mercruiser) hp I now have between 160 and 180 hp and a more efficient propulsion system. Did I really aim too high when I targeted 22 knots?

A forum member from Turkey (Cemberci) calculated from the supplied data that 14x17 should be the prop size and because I found no cause to contradict him, so that is what I ordered. One clockwise, one counter-clockwise, delivery time 3 months for the latter.

Feeding air in the tunnels and using surface props is an alternative I am seriously contemplating, but with the previous blunt tunnel entrance there was air draw in, resulting in lots of vibration. That might be better with more prop blades but I read a lot about vibrating surface props.

[baeckmo]

A quick browse through the previous links on this project shows that a number of writers have in beforehand correctly pinpointed the issues that would cause trouble. I believe it was ”Ad hoc” who described it as a backwards engineering project. And trouble there were….. . There is no chance that a simple pitch change will be medicine enough for this patient! That much said about the specific tunnel project, I feel that a few arithmetic exercises on the subject of this tread, namely changing pitch, might be of general interest. I might add that we regularly twist and beat propellers and jet impellers in our shop here to suit special demands, so I am familiar with the process…… .

First to the importance of precision regarding blade shape:
Using standard algorithms for propellers of the Wageningen B-type, we get the following results for a 14 x 17 propeller, 3 blades BAR 0.55 in a decent tunnel design (wake factor ~8%), 1500 rpm´s and speed 12 knots: with a shaft power consumption of 44,2 hp it will produce 3132 N of thrust. (The wake factor in CDK´s tunnels is rather something like 30 to 40 % ).

To get an idea on the consequenses of a correcting job done by an inexperienced sledge hand, we assume that 5 % of thrust are lost due to inaccuracy in shape. In order to maintain the same boat speed as before, the engine will have to operate at increased rpm´s. In our example, the operating point is changed to 1526 rpm, where the shaft power consumption will increase to 47,2 hp. With power losses in the transmission, the total power difference will be abt 4,3 hp.

Now, this amounts to a difference in tacho reading of 52 rpm (gear ratio 2:1), which is easily hidden among other dissimilarities between testing runs before/after, not to mention tacho inaccuracy. But make no mistake here, the oil sheiks will still charge you for the increased energy consumed!! This is the dilemma in propulsion technology; the real outcome is very difficult to measure. Nobody has ever seen a horsepowerhour or scratched its belly, but we all have to pay for them! If this one engine spends 200 running hours at this specific load, it will have to produce 860 hp hours extra. With a diesel (indirect injection) specific consumption of 0,24 l/hph, at a cost of ~1 Eur/liter, the total extra cost for this short period is 206 Eur.

This is more than the cost, including VAT of a brand new 14” standard propeller. A professional repitching or minor repair would come at about 60 Eur in our region. With this in perspective, I do not think that a novice to propulsion technology should reach for a hammer as a first priority………!

Then to the leading edge problem when reducing pitch:
Let us just check what happens on the mean effective radius, which is ~0,7 x tip radius. For our 14” prop, the mean radius is taken as 0,125 m. On this radius the chord is about 0,180 m. With a nominal pitch of 0,432 m (17”), the blade has an angle (leading edge to trailing edge) to the disc plane of 28,9 degrees. With 16” pitch this angle is 27,5 degrees. This amounts to an axial difference of close to 4 mm. When the leading edge is rolled down to achieve this change, I often see that the job is done on the first 25 % of chord. In our example this would be something like 40 to 50 mm.

Done this way, the local leading edge incidence will be about 23 degrees instead of the 27,5 we aimed at, often causing a pressure reversal on both sides at nominal chord incidence. The blade chord will have a hook near the leading edge which is a very bad profile, particularly in a cavitating environment, where it will be prone to pressure side cavitation. On the other hand, a profile with a hook in the exit region, due to pitch increase, will cause the blade load to move aft on the blade, whis is beneficial from cavitation point of view. This is my reason to recommend you to start with a pitch on the low side when there are uncertainties regarding dimensioning.

And finally: Can the blade be twisted around its chord center line?
With our 14” example again, the hub dia is about 22 to 25 mm. The blade chord is clamped to the turning lever on a radius of about 50 mm. All the plastic deformation takes place between those radii. A fraction of the total propulsion power is spent in this zone, so its shape is not critical to total performance. The result is a blade with its original chord shape in order, good efficiency and no vibrations. It is carried out as a standard procedure on thousands of propellers so far, in sizes up to at least 20” in various materials.

[CDK]

Thank you Beackmo, that was very informative, but it also raises some questions.
The term "wake factor" confuses me. There is a Froude's wake factor, a Tailor's one and even a wake factor for vehicles in dense traffic.
In most technical papers, a wake factor is assumed, not calculated or measured and the assumed value is always small, like 0,05 or 0,1. That makes me a bit suspicious about prop calculations because if the assumption is wrong, so will be the calculated result.

I quote from the Propeller handbook, by Dave Gerr from Int'l Marine:

....Determining Wake factor (Wf) as a function of block coefficient - Displacement Vessels.

Vessels with a higher block coefficient are fuller-bodied (tubbier). Accordingly, water flows around their hulls less easy and their wakes are greater than those with finer, more slender hulls. As you can see, the smallest wake factor, and thus the largest difference between V and Va, appears for craft with large block coefficients....
Then follows a diagram where the wake factor is between 0,6 and 0,95, the highest value belonging to a slender hull with twin props.

I assume you define wake factor quite differently because you qualify my tunnel design as having a wake factor of 30%, possibly more. Somehow I get the impression that the higher your factor, the more power is lost.

There is also a minor item at the end of your post, where you assume the hub diameter to be approx. 22-25 mm. You probably mixed diameter and radius: the shaft is 1 inch, so the hub diameter will be about 65 mm at the front and 45 at the back, averaging 50 mm.

[fasteddy106]

CDK, I know this may sound silly, but. Is there a chance the propshaft angle is off? I had this happen on an outdrive.The way it was mounted in the transom caused the prop to be slightly aimed at the mud all the time. The speed increased magicly when I corrected this as the engine stopped trying to push a bathtub on end through the water and allowed the boat rise up on plane. I know this sounds simple to all you engineering guys but hey, it's worth a look.

[tom28571]

One thing that seems to be overlooked here is the relationship between the "hump" you are stuck in, the "hull speed" and the waterline length of your boat. When the bow rises up, the waterline length goes down and takes the hull speed with it. Often more power makes the bow rise more, making the hump problem worse unless the is enough thrust to propel the boat over the hump wave.

As all racers of small hydroplanes know, the solution is to get the bow down. They do that by leaning their weight as far forward a possible. Bigger boats do it with trim tabs to force the stern up and thus, the bow goes down. This not only increases the "hull speed" but reduces the bottom loading on the hull contacting the water and reduces the induced drag so the boat can get its butt out of the hole.

You can test this very easily and cheaply by temporarily adding wedges under the stern. You can make them out of wood and attach with a sealant that can be removed. I have done it on smaller boats with wedges attached with tape. If it works, install some trim tabs. From your drawing, the wedges/tabs will have to be placed out at the chines.

[baeckmo]

CDK, you are correct about my writing dia instead of radius when discussing blade adjustment, thanks for the note! As for the wake factor, there are also different engineering cultures; in "my world" an 8% wake means that the propeller is working in a mean velocity field 8% slower than the main flow (=vessel speed). Consequently, when I guess that you have a 30% wake factor, it means that the inflow to the prop is 7 knots when the boat is doing 10 kn.

If we check your prop with a humble 20% wake (my definition....), it would draw about 53 hp and need a blade area ratio of 0,69 in order to keep cavitation within 10 % of the blade surface. In this condition the incoming flow one dia in front of the prop needs a throughflow area equal to over 18" in diameter. A correct propeller tunnel must be adapted to the flow field of the propeller!!

This operating point means an engine torque of 132 Nm (incl. transmission), which I believe is more than you get from the VW engine. The prop is certainly cavitating, otherwise your engine would have topped at a lower rpm.

If you could produce a basic power curve for this engine we could use the balance (propeller power)/(engine power) to find a likely wake factor for your tunnel, but at this stage, it is only of academic interest! (Btw, is this the six cyl engine? What exhaust manifold is used? Std watercooled ones are inferior to car manifolds in terms of power!)

We have built a couple of workboats with "external" tunnels here; I will check a few old files and see if there are any photos for you. If so, I propose we continue the discussion where it belongs; in the "DIY tunnel" thread. OK with you?

[CDK]

One thing that seems to be overlooked here is the relationship between the "hump" you are stuck in, the "hull speed" and the waterline length of your boat. When the bow rises up, the waterline length goes down and takes the hull speed with it. Often more power makes the bow rise more, making the hump problem worse unless the is enough thrust to propel the boat over the hump wave.

As all racers of small hydroplanes know, the solution is to get the bow down. They do that by leaning their weight as far forward a possible. Bigger boats do it with trim tabs to force the stern up and thus, the bow goes down. This not only increases the "hull speed" but reduces the bottom loading on the hull contacting the water and reduces the induced drag so the boat can get its butt out of the hole.

You can test this very easily and cheaply by temporarily adding wedges under the stern. You can make them out of wood and attach with a sealant that can be removed. I have done it on smaller boats with wedges attached with tape. If it works, install some trim tabs. From your drawing, the wedges/tabs will have to be placed out at the chines.

The prop shaft angle is between 6 and 7 degrees, so there is an upward component that somewhat reduces the tendency of the bow to rise. But you know that getting into plane without changing the hull angle is impossible, even with sterndrives fully trimmed in, the bow will come up.
I always had trim tabs on this boat while it was powered by Mercs, from my calculations with the tunnels I reckoned they could be discarded.

[tom28571]

The prop shaft angle is between 6 and 7 degrees, so there is an upward component that somewhat reduces the tendency of the bow to rise. But you know that getting into plane without changing the hull angle is impossible, even with sterndrives fully trimmed in, the bow will come up.
I always had trim tabs on this boat while it was powered by Mercs, from my calculations with the tunnels I reckoned they could be discarded.

What is really impossible is knowing the details of someone else's boat from posts on the forum. Of course, some up trim of the hull is necessary to develop dynamic lift. Too much weight and insufficient power (make that thrust to include wrong props, etc) also makes it impossible to plane. Trim tabs do introduce a lot of drag and need power to overcome that but they will often allow a boat to plane that otherwise would not. It's all about lift to drag ratio and how to balance the under boat stuff to maximize that. Sometimes you need to increase one at the expense of the other and sometimes vice-versa. I don't think any of us know which is your problem, assuming there is sufficient thrust available. With enough thrust, most problems can be overcome but it is a poor way to design.

[TollyWally]

Tom,
Could you expand on the trim tab and bottom loading thing. If this is not the appropriate thread I would gladly start another one. Does the surface area of the trim tabs help to lower the bottom loadong by supplying more surface area? Ever since reading about your Bluejackets, I've had the notion of large trim tabs increasing the surface area and lightening the bottom loading. I do like the attributes of those boats of yours. If I didn't already have one mistress and had more time I'd love to build a 28'.

[tom28571]

Tom,
Could you expand on the trim tab and bottom loading thing. If this is not the appropriate thread I would gladly start another one. Does the surface area of the trim tabs help to lower the bottom loadong by supplying more surface area? Ever since reading about your Bluejackets, I've had the notion of large trim tabs increasing the surface area and lightening the bottom loading. I do like the attributes of those boats of yours. If I didn't already have one mistress and had more time I'd love to build a 28'.

Wally,

Trim tabs are actually high drag things. That they can often get a boat on plane that would not otherwise do so only says that the boat was poorly set up for planing to begin with. Usually it is because the boat is very heavy. A heavy boat creates a larger bow wave that is harder to get over. With enough power, such boats do get over the bow wave and plane. Adding lift aft can reduce this bow wave height by increasing the waterline length. Whether the boat planes or not depends on whether the total drag is lesser or greater than before, including the reduced resistance of the longer and lower bow wave. I suggested that CDK try the wedges because that is an easy and cheap test to see if that was the reason for his boat not planing.

The trim angle of a planing boat will always adjust itself to produce the amount of dynamic lift needed to support the boat at that speed.

There are volumes written by experts on the aspects of planing boats.

Edited to add: I know that I have not answered your question completely. Every element of the boat is interrelated to every other and making simple statements is often very misleading.