Keel modifications

[RAMMYBOY]

Greetings All, I wish to swing a bigger propeller on my boat. To achieve this I need to move the stern tube down about 2", build up the skeg 2" and refit the keel band which runs from skeg to the rudder base mounting plate. Engine will be realigned once the above has been completed. Can anybody point out any pitfalls or recommend any books to help with this project?
Many thanks Rammy

[gonzo]

To start with, calculate the angle of the shaft. It should not be more than 15 degrees from the waterline when running. You need to add the angle of attitude at speed to the angle of the shaft from the static waterline.

[RAMMYBOY]

Thanks gonzo I"ll make that my first task then, a lot easier to do now with the boat still in the water I think!

[RAMMYBOY]

Greetings, The angle of the shaft is 5 degrees. I have contacted a couple of prop people with mixed results. Anybody offer any advice as to optimum size please? Here are the details.
L.O.A. 27ft.
L.O.W. 24ft
BEAM. 9ft 2"
WEIGHT. 11,200LBS about 5 English tons.
TOTAL B.H.P. 120
MAX R.P.M. 2800
GEARBOX RATIO. 1.21

Current speed is 8knts WOT. I had a new prop made and was promised 12knts. This never happened. Current prop is 4 bladed 14x12.5. Any thoughts greatly appreciated as I seem to be wasting my life (and cash) on this problem at the moment.

[Kay9]

Um, I could be wrong here but Hull speed on this boat is about 6-8 knts I dont think your going to ever get 12 knts.

source http://www.sailingusa.info/cal__hull_speed.htm

K9

[RAMMYBOY]

Hi Kay, My boat is an Aquabell, which is semi-displacement. I know of 2 others that run at 20+knts. These are stern-driven, which I belive mine was at some stage. The boat is 1980, The engine is a Perkins 6354, somewhat later I believe.

[Kay9]

Well, like I said "I could be wrong".

Ill get you some info on prop sizing.

K9

[Kay9]

As promised:

http://www.propline.com/Propeller-General-Information/Propeller_Sizing.htm

http://www.acepropeller.com/prop-spec.asp

http://www.tacomapropeller.com/8467/Inboard-Propeller-Size-Help.html

I hope this helps

K9

[RAMMYBOY]

Much appreciated K9, I will run the figures through them. Rammy.

[Rick Willoughby]

Greetings All, I wish to swing a bigger propeller on my boat. To achieve this I need to move the stern tube down about 2", build up the skeg 2" and refit the keel band which runs from skeg to the rudder base mounting plate. Engine will be realigned once the above has been completed. Can anybody point out any pitfalls or recommend any books to help with this project?
Many thanks Rammy

It is a very heavy boat for a 14" prop so fitting a bigger prop will improve efficiency.

The performance figures you have provided do not quite add up. You should be able to get better than 8kts even with that dinky prop if the prop is actually spinning at 2800/1.2 rpm. Are you certain you are getting full engine rpm? I estimate it will be doing around 1900rpm rather than 2300rpm if it is the size you say.

The current shaft angle is not particularly severe. Even a larger prop will operate at high slip so the moderate shaft angle is not a great concern.

Rick W

[Rick Willoughby]

Another thought.

Does the boat sit on the waterline shown is it is trimmed bow down with the Perkins rather than the stern drive. If it is trimmed badly it might not be getting any lift so it will just dig a big hole. This should be obvious.

Is it something you have observed?

If this is the case then improving the prop will produce a bigger hole without much increase in speed.

Rick W

[RAMMYBOY]

It is a very heavy boat for a 14" prop so fitting a bigger prop will improve efficiency.

The performance figures you have provided do not quite add up. You should be able to get better than 8kts even with that dinky prop if the prop is actually spinning at 2800/1.2 rpm. Are you certain you are getting full engine rpm? I estimate it will be doing around 1900rpm rather than 2300rpm if it is the size you say.

The current shaft angle is not particularly severe. Even a larger prop will operate at high slip so the moderate shaft angle is not a great concern.

Rick W

Hi Rick, The shaft rpm are 2314 according to Castle marines calculation. There was no serial number on the gearbox (borg-warner velvet drive, on a vee-drive) I calculated the ratio with an optical tachometer on the shaft and my new vetus dashboard tachometer. The nearest ratio in the borg-warner book was 1.21 . I have tried different prop diameters on Castles site and they all say that the ratio is not ideal,although they do not tell me the ideal ratio. It looks as though I may have to alter or replace gearbox. What do you think? My main concern at the moment is deciding on the approx diameter of ideal prop 16" 17" 18" 20"? so I can work out new stern tube position. Thanks for your interest. Rammy
P.S. my taco shows 2800 rpm

[RAMMYBOY]

Sorry, just noticed your other post, She sits nicely on the water, engine is mounted aft of midship. Engine (back to front on vee drive) to allow a bigger wheelhouse I imagine.

[Rick Willoughby]

Having the boat in trim means she should be really starting to lift the bow at 8kts.

It should be possible to check the gearbox ratio by turning the motor and comparing rotation with the prop. I expect your motor tacho will be OK. Any increase in diameter will help. The bigger the diameter the better up to the point where the shaft angle is excessive. Aim for less than 15 degrees.

A large step up in diameter can be offset by selecting a smaller pitch or altering the gearbox ratio. If you can fit a 20" prop then the pitch will be about 10". A P/D ratio of 0.5 is less common than say 0.8 to 1.2. Hence, if the ratio is 1.2, it will be low for swinging a more suitably sized prop.

How do you measure speed? It would be good to be certain about the current operating point for your 14 x 12.5 prop as 2333rpm and 8kts. The trim of the hull and weight are other main variables. I would expect better than 8kts under the conditions you have given. Have you any idea of the trim when at 8kts.

Rick W

[Ad Hoc]

Looking at my charts for boats of this type, engine etc etc, if your RPM is 2300-ish and the power is 120-ish...then you're looking at around 16" diamater. But.....!!!

[RAMMYBOY]

Thanks again for you replies. I am pretty certain about my top speed, I have 2 gps-plotters on board. I will try and get a photo of my boat underway to have an idea about the trim angle. Playing around with the proc-calc site, I entered the 5 different ratios available for the borg-warner, they all came up with a top speed of 12.42knts with prop diameters from 15--28" I will probably go somewhere in the middle. One thing I noticed was 3 of the calculations called for 4 bladed props and two for 5 bladed ones. Any idea why a three bladed one cannot be used? Finding this an interesting project--lots of questions unasked yet!!!
Rammy

[Ad Hoc]

Basically because you have such a low g/box ratio. It is generally bad practice to use a high rotating prop to drive a slow boat. The greater number of blades help to increase the BAR (blade area ratio) and delays cavitation; it reduces the thrust loading on each blade.

[RAMMYBOY]

Basically because you have such a low g/box ratio. It is generally bad practice to use a high rotating prop to drive a slow boat. The greater number of blades help to increase the BAR (blade area ratio) and delays cavitation; it reduces the thrust loading on each blade.

The highest ratio I tried was 3.4 this called for a 28x24" prop with 5 blades. Is the perkins 2800 a high rpm or about average?

[Ad Hoc]

I don't know what input the program uses, but the numbers are all wrong. That is the program with these "simple DIY" prop design programs....gives you answers, but not real ones, since it doesnt know what a real answer is!

You have an rpm of some 2300-ish...at 120-ish hp...the diameter prop best suited to this is roughly 16", with a pitch around 7~8". This is based upon the data you have provided. As i noted above, your boat is slow and your shaft rpm is high..bad combination!

When you says the highest ratio of 3.4...ratio of what?..this is not clear.

[RAMMYBOY]

Sorry, I made a typo there. The ratio I entered from the borg-warner book was. 3.14: 1.00
The other ratios are
1.99:1.0
1.51:1.0
2.49:1.0
1.21:1.0
0.96:1.0

The nearest to your calculation it came up with was a 15x9 prop with 5 blades a DAR of 88% a slip of 0.31 pitch ratio 0.60. This was with my current ratio of 1.21:1.0.
Intresting comment about the calculation site, how else can I go about these calcs?
Rammy

[Ad Hoc]

Well, ask yourself this question...How many boats of your size/type/power have props of 28x24 going at 12+knots?

Whilst we can debate maybe the pitch of diameter should be slighter higher or lower, accordingly to the calc's....but, is it a realistic meaningful result? Ergo, how many boats do you see with a 28x24 prop?

Naval Architecture (designing boats) is not an exact science. There are disciplines within, which are such as: mechanical, electrical and structural, some aspects of hydrodynamics too. However, as a complete package, the "exactness" often falls down in reality. ..ie a boat is greater than the sum of its individual parts. Only experience can tell you this, not numbers spued from a program. In the absence of experience, look around you and your marina/magazines etc..what do you see, hence the question about how many props of 28x24 do you see? So, some self analysis can be done to obtain a better idea.

So, a 28" prop is very very large for a boat your size and power, just go to your local marina and check.

There are only 3 things to consider when selecting a prop
1) The speed of water in way of the prop (different for different hulls)
2) Horse Power at the prop
3) Prop RPM.

The hull, can only go so fast with so much power given and also the shape of the hull shall dictate the speed too...etc etc...so, many things to consider, which a prop DIY program wont tell you.

PS..forgot to add.
If your engine has an even number of cylinders, ie 2,4, or 6 cylinders, for example, best to select a prop with odd number of blades. And vice versa.

[RAMMYBOY]

Thanks for the detailed reply, starting to make sense of it all now, I think! I have tried to contact similar boat owners with limited results. Most have bigger engines and set up with a leg, which I do not want. One I know of has 175bhp with a 14" prop and achieves 14knts. Not very efficient methinks. I appreciate it is not an exact science, like my trade joinery, experience is crucial for a good outcome. The perkins has 6 cylinders. My choice is a 16x8-9" 3 bladed prop. may get one off the shelf £2-300 or a custom 5 bladed one, considerably more I imagine. I won"t hold you to it, but which would you chose? Hey I"m getting a collection of props now, so one more won"t hurt!!
Rammy

[Ad Hoc]

Before you start selecting your prop, your original question was about just simply selecting a bigger prop.

So, firstly, back to the beginning..why do you want a bigger prop?

Is it simply because the current performance of your boat is less than expected?..or is there another reason?

Just want to establish the basic facts first. Since this thread has deviated a tad and not sure what you really want as your priority.

[RAMMYBOY]

I just want a bit more speed, I fish offshore at times and the journey time can be a chore. 14.5" is the maximum I can swing at the moment, and it just (looks wrong). I love this old boat and will re-engine with more horse-power (which I assume will need a bigger prop) when funds allow. I am pretty practical, and my son is an chief engineer on a super-yatch so hope to do most of the work between us. That"s about it, I enjoy boating and solving boaty problems.
Rammy

[Ad Hoc]

Ok, so it come down to some basics.

Your hull has a hp/ton around 24, this should get you around 11~12 knots may be a tad more. However, the length displacement ratio of just over 4. This is low.
The L/B ratio is also low...all in all, which means even though the Froude number (at 12knots) is around 0.7, the hull is struggling to go faster. So even though the hp/ton suggests a predicted speed, doesn't always mean it can be achieved, owing to that already mentioned.

However, this and the following really depends upon 3 factors
1) Is the displacement correct?...how ahve you checked it to confirm the 5tonne?
2) Is the Hp correct, again, how can you confirm this is correct?
3) Is the RPM correct, again, how can you confirm this is correct?

In each case, you need actual figures, not figures taken or given from manufactures. You need to actually measure it.

Because the performance of your boat AND the prop is governed by the above. Any slight deviation, say 100Hp and not 120HP, will effect the selection.

The main "issue" is that your engine power is high for such a small heavy boat, which suggests, as noted, the hull parameters are not desirable for going fast. This is not to say you can't, since you ahve a "chined" hull...but it is really struggling.

[RAMMYBOY]

Hello again, The last time I had boat lifted, the hoist operator said between 4+ 5 tons. I opted for 5 tons to allow for fuel, water and passengers.
The horsepower is only the given one in the manual, compression tests were done and ok, engine runs and sounds ok. Can HP be tested away from the bench?
The RPM of engine and shaft were checked with an optical tachometer.
The boats tachometer is a new Vetus and confirms the optical taco for engine RPM.

[Ad Hoc]

Hmmm...4 or 5 tonnes...that is a difference of 20%!!!!, on any boat, this is not a small amount...you really need to establish the lightship and then what is your max deadweight to get an accurate full load.

[RAMMYBOY]

Don"t really know where to get an accurate weight, there is a crane on a river nearby, but he can only set his jib alarm, so not the most accurate I would think. The 40ton hoist operator in my harbour says he cannot give a totally accurate weight due to the configuration of the sliding strop mechanism. This has 4 weight dials on it, maybe he just doesn"t know to use it. So I am a bit stumped there. How about the HP issue, any thoughts on that without removing the engine for testing?

[Ad Hoc]

Well, a quick way to establish the displacement(or mass), although not wholly accurate on small boats, is to read the draft marks and check against the hydrostatics of the stability book. This assumes of course you ahve a stability book? You would need a hydrometer to confirm the salinity of the water too.

Do you have the engine performance curves, from the manufacturer??

[RAMMYBOY]

I do not have a stability book.
I do have the performance curves.

[baeckmo]

Rammy: first, as Ad Hoc has remarked, there is something that doesn't add up in your rig here. A preliminary calc on your propeller shows a remarkably low efficiency, further reduced by cavitation due to high load.

1/ There are a few versions of the Velvet V drives that do not have the same ratio fw and reverse. There is one with 1.21 fw and 1.33 rev f.ex. Fw direction should be cc-wise (if I recall correctly...). Please check prop shaft rpm,s with a tacho!

2/ There are versions of the Perkins 6354 with max 2400 or 2600 rpm. With hot engine and in neutral gear, check maximum no-load rpms (increase throttle gradually!), that might indicate which setting you have.

Unfortunately, I can not see the propeller and its "environment" in the pic; there may be a wide chunk of keel destroying inflow to prop as well. It's a nice boat you have got there, and she certainly should have better performance than 8 knots. If you could provide answers to my quests above, we should be able to find a solution, so don't rush away bying a new screw yet!!

[Rick Willoughby]

Hi Rick, The shaft rpm are 2314 according to Castle marines calculation. There was no serial number on the gearbox (borg-warner velvet drive, on a vee-drive) I calculated the ratio with an optical tachometer on the shaft and my new vetus dashboard tachometer. The nearest ratio in the borg-warner book was 1.21 . I have tried different prop diameters on Castles site and they all say that the ratio is not ideal,although they do not tell me the ideal ratio. It looks as though I may have to alter or replace gearbox. What do you think? My main concern at the moment is deciding on the approx diameter of ideal prop 16" 17" 18" 20"? so I can work out new stern tube position. Thanks for your interest. Rammy
P.S. my taco shows 2800 rpm

Rammy
You might find this thread of interest:
http://www.boatdesign.net/forums/projects-proposals/hull-modifications-23205.html

It is not the same as what you are doing but the boat is a somewhat scaled up version of your Aquabell. Blizzard has the advantage of shallow tunnels so it can swing big props without going really steep on the shaft.

The main thing to note is the size of the props and the weight of the boat. Roughly twice the weight and two big props that achieve good efficiency. Drag on this boat is less than half that on the Aquabell so prop loading is much much lower.

Your 14" prop will achieve around 40% efficiency. At 12kts you will be getting toward the drag hump that is not much less than what would need to be overcome to get fully on the plane although you will have nowhere near enough power to do that. However your prop still has to generate roughly the same thrust as that required by your hull to plane. A single 14" prop is a very inefficient means to achieve that - 60% of the fuel you are burning is simply churning water.

The other thing to note is the performance gain on Blizzard by cleaning up the underwater bits. If your long keel is poorly streamlined it will add significant losses.

Rick W

[Ad Hoc]

I'd be interested in your opinion baeckmo.
From the pic's, i'd say he's got a wake of around 10%-ish. (maybe ~15% with the size of the skeg)

But the speed of the boat i reckon is not wholly down to the high loading on the small prop, (hence a larger prop is required)... the characteristics of the hull does not lead itself for big gains, just small ones. The form factors are not ideal.

Having said that, I think the reduction ratios posted of 1.99 or even 2.5 would improve matters considerably, rather than just changing the prop diameter. A combination of the two are required. This of course would lead to a much larger prop diameter than originally envisaged!

[baeckmo]

Yes, Ad Hoc, I certainly agree on your reflections regarding gear ratio and wake (hence my note on picture). Using a wakefactor 0.15, a shaft depth of 0.7 m, shaftspeed 2314 rpm and calculating for Wageningen B 4 blade with BAR 0.69, the noncavitatingresult is:

Ja=0.26, eta=30%, Pprop=136 hp, T=8458 N, Sigma07=0.16.

In order to limit cavitation to ~10% of the blade area, a BAR of 1.83 would be required (30% cav needs BAR 1.13)!

The available shaft power is ~110 hp, provided the engine is healthy and drinking good diesel fuel. The sc "green diesel" or "envirodiesel" or whatever is reducing engine output power some further 8%. Checking a "generic" engine power characteristics, this engine would have managed ~2640 rpm with this prop, if it were operating cavitation-free. Since the owner reports 2800 rpm, my conclusion is that it may be working on the governor rpm limit while heavily cavitating.

Obviously, the thrust is seriously reduced. I won't spend time to dive into detailed calc on cavitating performance now, but just using a "tau/sigma"- rule-of-thumb diagram, I would not be surprised if the effective thrust were down in the 5100 N region. First step is thus to reduce blade speed/increase disc area.

One more indicator for propeller load: the Taylor Bp factor would be over 200 if the full herd of horses were kicking around. This simply can't work! The honoured sir Charles Algernon Parsons and his tandem propellers comes to my mind..........!

Now I would like to have a response to my previous notes before we start any serious brainstorming here!

Come to think of it: It might help in the selection if you Rammy could provide a set of speed/rpm observations; say from 5 knots up in steps of 200 rpm.

[RAMMYBOY]

Thank you all for your interest, the hull modification thread makes my problems look minor in comparison! I will check the engine RPM again, pretty scary being that near to the old girl in full flight!! I will check the shaft RPM again(even scarier with head under the deck!) also check reverse ratio. I will get some pictures together of boat static, underway and stern gear. I am not looking for a quick fix, so won"t be rushing out to buy a new prop just yet! Plan to carry out this project in the spring, so am glad I started the research in plenty of time. Rammy

[Ad Hoc]

Rammmyboy
"..I am not looking for a quick fix,.."

Perfect. That is the most sensible retort anyone can give. Problems such as yours requires more than a one liner and a pretty picture reply from anyone. So I'm pleased to read this.

Baeckmo will have far more data at his fingers tips than I. (I have to wipe the dust of my old charts!!). So, once you're able to overcome your scary adventure :P, I'm sure we can get to the bottom of this. Just takes patience, which it appears you have plenty of :)