Help With Inboard Shaft Angle

[rich99uk]

Hi please see the pictures as you can see the shaft angle is rather steep well i think so anyway. I wanted to know what angle it should be or how to work it out????
I will be cutting of the keel as this stops it plaining.
Do you think that by makeing the angle less steep it will perform much better?
The boat never originally had a inboard and shaft that was put in by us. But obviously we got the angle wrong so any sugestions or help would be great !!

http://i970.photobucket.com/albums/ae189/kingrich2009/000_0140.jpg

http://i970.photobucket.com/albums/ae189/kingrich2009/100_0558.jpg

http://i970.photobucket.com/albums/ae189/kingrich2009/IMG_1697.jpg

http://i970.photobucket.com/albums/ae189/kingrich2009/100_1016.jpg

http://i970.photobucket.com/albums/ae189/kingrich2009/100_1007.jpg

http://i970.photobucket.com/albums/ae189/kingrich2009/IMG_1694.jpg

[rich99uk]

I have decided to completley remove the shaft and keel and putthe shaft in on less of a angle and with no keel.
I am hoping to use a v drive as i cant move the engine eny furthey forward.
i justdont know what the angle should be ??

[CDK]

I won't bother you with math but look at it this way:
With the shaft vertical, the boat goes up like a helicopter. With the shaft at 45 degrees, 50% of the force moves the boat fwd and 50% try to lift it.
With the shaft horizontal, all energy is used for propulsion but you have mechanical problems.
So the trick is to position the shaft as close as possible to the horizontal axis. I think any angle under 20 degrees is acceptable, under 15 is good. But there are also boats with a shaft angle like yours, but they do not plane.

For a 12 degrees angle there are standard parts on the market.

[daiquiri]

And what is the shaft angle in this case? It is not easy to estimate it from the photos (looks like something around 20°, is it so?).

You should give some more info about you boat's characteristics, if you want to obtain a useful advice. Boat waterline length, beam, operative weight, engine characteristics, gear ratio, propeller diameter and pitch is the minimum info you should provide.

Also, have you already done a test run? If yes, what was the max speed obtained at full throttle (measured possibly with gps, in calm water), at what rpm, and what was the boat's trim angle? Did you notice some particular engine fumes behind the boat? Or any excessive vibrations?
That would be useful.

[fcfc]

I am just asking, without any useful answer.

A inclined shaft have generate a vertical component. But up to what point the lift or trim change generated is beneficial to propulsion ?

Given the "profile" of skeg and rudder, is shaft inclinaison that much important ?

Up to what point an angled transmission loss cover more horizontal shaft gain ?

[anthony goodson]

Rule of thumb 5degrees 99.6% 10degrees 98.5% 15degrees 96% of pwer transfered to forward motion .So at 15degrees you have roughly 4% trying to lift your stern and bury your bow

[CDK]

Rule of thumb 5degrees 99.6% 10degrees 98.5% 15degrees 96% of pwer transfered to forward motion .So at 15degrees you have roughly 4% trying to lift your stern and bury your bow

I think you need a new thumb.
Vertical and horizontal components follow tan and cot. At 10 degrees the relation is 97/3 %, at 15 degrees 92.8/7.2% and at 20 degrees 87/13%.

[Joakim]

I think you need a new thumb.
Vertical and horizontal components follow tan and cot. At 10 degrees the relation is 97/3 %, at 15 degrees 92.8/7.2% and at 20 degrees 87/13%.

If you assume that the thrust is at the direction of the shaft (which is not absolutely accurate), you get the following for 1000 N thrust

deg hor ver
0 1000 0
10 984 174
15 966 259
20 940 342

These are calculated with sin and cos. If you keep the horizontal thrust constant, you can use tan and then get

deg hor ver
0 1000 0
10 1000 176
15 1000 268
20 1000 364

Where did your numbers come from? The perpedincular forces do not sum up to 100%, you need pythagoras.

[daiquiri]

I am affraid that there is no simple formula for the calculation of inclined propeller's characteristics. Cavitation can change significantly the Kt and Kq values for the same shaft angle and the same advance ratio.

There are several models available for props in oblique flows. For non-cavitating regimes Gutsche's model seems to perform pretty well, but it cannot be expressed as a simple multiplication by a single sin, cos, tan or ctg function, since it is based on the blade element theory. Athe general result of that model is that both Kt and Kq increase with shaft angle, since the effective advance ratio J (perpendicular to prop blades) is smaller than in case of shaft aligned with the flow. Therefore, one could expect that the prop efficiency can in some cases even increase with some reasonable shaft inclination. But again, strictly speaking, the model is valid for non-cavitating props only. I remember reading some reports which actually indicate the increase of prop efficiency with shaft angle of attack. If I manage to find them, I'll post the info here. Until then, consider it as just a theoretical result.

Most propellers will work with more or less cavitation developed over its blades, so these results can change considerably. Cavitating propellers experience a decrease in both Kt and Kq as the shaft angle increase, as well as a decrease in efficiency. I am not aware of any validated mathematical model for cavitating propellers in oblique flow, unfortunately. Hopefully someone else will know better.

[CDK]

If you assume that the thrust is at the direction of the shaft (which is not absolutely accurate), you get the following for 1000 N thrust

deg hor ver
0 1000 0
10 984 174
15 966 259
20 940 342

These are calculated with sin and cos. If you keep the horizontal thrust constant, you can use tan and then get

deg hor ver
0 1000 0
10 1000 176
15 1000 268
20 1000 364

Where did your numbers come from? The perpedincular forces do not sum up to 100%, you need pythagoras.

Joakim, good to see I am not the only using math occasionally.
My figures are just approximations, obtained by playing with tan and cot from a table, and of course a calculator. Your results are also not correct.
With a 45 degree shaft angle, both tan and cot are 1, so the force vectors have equal length: 50/50. I think we can accept that without calculations.

At 20 degrees angle, the vectors cannot be 1000/364 or 64 and 36%.

The actual calculation is impossible without data like waterline, displacement and hull shape because the whole package changes position as a result of the vertical vector. But how large that change is depends on these unknown factors.
And then there are also the peculiar models diaquiri has mentioned, where tilting a shaft increases performance.

Like most engineers, I think my figures are reasonably accurate until the opposite is proven.

[Joakim]

Your results are also not correct.
With a 45 degree shaft angle, both tan and cot are 1, so the force vectors have equal length: 50/50. I think we can accept that without calculations.

At 20 degrees angle, the vectors cannot be 1000/364 or 64 and 36%.


Please tell me, what would be wrong with my "results"? What do you mean by that comment about 20 deg?

A 1000 N at 45 degrees is equal to 707 N horizontal + 707 N vertical force.

[baeckmo]

Naah, c'mon rich99, give us some numbers as requested by daiquiri; what power at which rpm, what gear ratio aso!! Otherwise these guys will compare their thumbs straight into the eternity.......and no one will remember where it all started!

[bit]

Daiquiri
Therefore, one could expect that the prop efficiency can in some cases even increase with some reasonable shaft inclination. But again, strictly speaking, the model is valid for non-cavitating props only.
I do not understand...
Ciao

[rich99uk]

Ok guys cheers for your comments, Here is some more data for you.
I curentttly running at 80hp at 4500 Rev 2-1 gear box 16 X 11 Prop. full throttle i can do 10 knots and at this point i think i am on the plane as the water rushes through the tunnels under the boat and the back of the boat bottom of the transom is flush with the water. I mean the back of the boat is dry. as soon as it gets a little choppy i come of the plane. i have to be almost open full throttle to get there.
When there is a stong tide it can slow me down to 6 - 7 knots.

But this is all changing, when i change the angle of the shaft i will be runnning with 120HP and using a v drive instead of straight shaft ration of gearbox will be 2:1 i think. And of course there wil be NO keel, Hopefully to make it a planeing hull.
weight of the boat is just over 2 ton so say 2 and a half.
What Other info can i give you Guys ???

[daiquiri]

I do not understand...
I have outlined the results of Gutsche's model. In my opinion it applies only to props which have not been optimized to account for shaft inclination.
Sorry, but i cannot elaborate more now. It is 00.30 a.m. and tomorrow morning at 5.00 I'll be leaving for a 3-day vacation (boy, I do need it...). So, more to come from me next wednesday. I can't wait to see what will happen in this thread in the meanwhile, but I do have like an idea...

[Joakim]

It's hard to see from the photos due to black paint, but it looks like this is not a prismatic planing hull. Chineline seems round and also the keel added seems to have rounding at the hull attachment. If that is the case, it is not worth trying to make it plane.

At 10 kn it is certainly not planing, but it is above pure displacement. If you need 80 hp to reach 10 kn with a 2 ton boat, it seems the hull is not a good semi-planing hull, which would only need ~40 hp for that.

Just roughly calculating it looks like your propeller is cavitating badly at full speed. This would make it's efficiency far from optimal and may also damage the propeller. How was the propeller chosen?

Did the boat originally have an outboard? How big? Not on the center line as in the photos?

You should still give us much more information like the dimensions of the boat and the speed at different rpm.

[CDK]

Please tell me, what would be wrong with my "results"? What do you mean by that comment about 20 deg?

A 1000 N at 45 degrees is equal to 707 N horizontal + 707 N vertical force.

The difference kept bothering me because I couldn't repeat my own calculations, so I drew the vectors.
I must apologize to Anthony Goodson's thumb. The loss in horizontal force is minimal, even with a 20 degree angle there still is 94%. Its the 34% lifting force that may be very unwelcome.
Some points for you Joakim.

[rich99uk]

Ok some more pics of the hull
Now the boat originally had two 150HP outboard engines on it but i dont know how it went with them on as i boat just the hull.
the beam is 6ft 6"
With regards the revs i cant tell u exactly but basicly its gradual as i accelerate i slowly get faster i must admit the last bit of throttle not alot happens doesnt make a differenceto my speed.
My theroy from what i can see at the monet is when i open her up the back of the boat falls so to speak the water is being pused from under the boat making a hole and the boat sits in it rather that being pushed along.
So would u say my boat is a non plaining hull ? Imean when i take the keel off that is.
I did a little experiment and cut some of the keel away and i noticed that when opening her up she didnt point so high it was like the keel was acting as a drag on the back of the boat pulling it down making the front rise.
Which still could be the case now and the angle of the prop would be even more increased.
What you think?
With regards the prop it wasnt calculated just a guess. But I wouldnt say it revs out and if it does its very minimal.

[Joakim]

So what are you actually trying to achieve? You modified the boat from 2*150 hp OB to 1*80 hp IB with a keel and now you want to make it plane again? If you wan't to make it plane, the natural choise is an OB or a stern drive. Those are designed for planing boats. IB with shaft installation and rudder increases drag very much at planing speeds and also the inclined shaft is far from optional at those speeds (= 20+ kn).

The keel you have build adds a lot of drag at higher speeds, but not that much at 10 kn.

If this is a planing boat (still not sure from the photos), 80 hp is not enough. 120 hp might be just enough to get it plane. If you don't have enough power for a planing boat, your are not able to get it over ~10 kn. The fact, that the boat had 2*150 hp refers to a planing hull or then someone was rather stupid...

If it is semi-planing, you should be able reach 12-15 kn at 80 hp and a bit more with 120 hp.

[rich99uk]

i want to be able to achive 15 knots anything else would be a bonus.
with your opinion what angle should i have the shaft to get the best performance.

[Joakim]

i want to be able to achive 15 knots anything else would be a bonus.
with your opinion what angle should i have the shaft to get the best performance.

I don't think the shaft angle is the primary problem here. More than 2 ton for only 2 m beam is very much. How long is this boat?

The boats that I know of reaching 15 kn with 80 hp at 2+ ton weight have a beam of ~3 m (10') and are specifically designed for that speed.

[rich99uk]

The boat is 21 ft

[TollyWally]

That's kind of a pig of a boat if it's 4500# or so @ 21 ft. Now you're running roughly 1/3 the hp through a wheel optimized for who knows what.


Check out Dave Gerr's prop book. Figure out how much hp it's going to take pushing the biggest wheel running at the slowest speed to get that boat moving and then adjust specs for your situation.

If that boat ran ok with twin outboards you'll be able to get it going with an inboard, you just need a better engineered drivetrain.

[anthony goodson]

Hi Rich
Have you 'flattened' and re-moulded the hull beneath and aft of the skeg/keel in order to try to create clearance for the prop ?
Don't misunderstand me ,this is a question, not a suggestion, it's difficult to see in the photos.

[rich99uk]

where the prop comes through the shape of the bottom of the boat changes and yes it goes flat but that is how the boat was built i didnt change that.

[anthony goodson]

Is there any sign of anyone having filled/mended a very large hole in the middle of the transom ?

[rich99uk]

yes me before i put a shaft on it i had a volvo 270 outdrive leg on it which did 14 knots with 80 HP. But i was always having problems with the leg so i done away with it.
However it didnt originally have the outdrive leg it had to outboards. I put thw whole there lol and put the volvo leg on myself.

[Joakim]

yes me before i put a shaft on it i had a volvo 270 outdrive leg on it which did 14 knots with 80 HP.

Why didn't/don't you tell all the information you have? You would get much better advices, if you would tell everything you know about the boat and not just drop extra information one by one.

E.g. it would be very helpful to know which gear ratio, propeller and rpm you had with the stern drive. I guess the engine was the same. At what rpm is it rated 80 hp?

[anthony goodson]

I am trying to answer your question regarding the planing ability of your hull.In your latest picture ,the one on the crane ,the hull appears to be a classic planing twin tunnel hull apart from a rather long and deep forefoot However Joakim quite correctly pointed out two areas of concern .He noted the apparent rocker on the chines and this " rounded area on the aft end of the keel." If you put your earlier pictures into Picasa and introduce some light you can see that the rocker on the chines is confined to the profile of the chines ,and should not cause pressure loss in the tunnels .but the cutaway in the keel is a mystery,and would not help the hull to plane, as the centre section the forward part of which is designed to help with the lift would lose pressure at that point.It almost looks as it was made to install a Hamilton jet.[it may not look like that in reality but it's how it looks in the pictures,]which are all we have to go on. On balance I think there is sufficient lift in the hull that it will plane but no doubt others will express their views.

[rich99uk]

Hi sorry Guys lack of info on my part, My bad sorry. It was a 2-1 ration leg i think the prop was 16x15 and yes same engine. 80 HP.
With regards the hull its weird because it goes in a V in the middle all the way the length the boat intill u get to the back where it gets shallower and turns into a flat bit. the back of the boat is shallower from the rest not alot of boat in the water there.
It had a massive splashwell which was part of the originall boat so defo was ment for outboards unelss they changed there minds when building it.
This history behind this boat is that it was a prototype from thosper Thorny croft for the hong kong Police in the war. from what i been told it didnt take off because it was to heavy. Only hear say dont know how much is true !!

[anthony goodson]

Bit like the Spruce Goose then ,although I am sure this postdates that and the war by many years. Vosper Thorneycroft bought PP jets ,Jets are very weight sensative ,I know it's a bit far fetched but could this be why there appears to be a piece missing from your boat.They are in your hometown,you may even have some mates that worked there why not ask around a bit more about the history.

[Joakim]

You still haven't told us what is the optimum rpm for your engine, but what we know now are two situations.

1. 80 hp, 2:1 gear ratio and a 16x11 3 blade propeller with rather small blades reaches 10 kn at 4500 rpm with the shaft, keel and rudder configuration in the photos.

2. 80 hp 2:1 gear ratio and likely a 16x15 propeller with a Volvo stern drive reaches 14 kn.

I think the key difference here is the propeller. Sure inclined shaft may reduce efficiency and keel and rudder have more drag than a stern drive, but not enough for a 10 vs. 14 kn difference. Probably your boat has its worse drag around 10 kn.

As I said earlier, the propeller seems to be cavitating. Without cavitation it would not rotate more than ~2000 rpm (shaft speed) at 10 kn and 80 hp. What happens to thrust and efficiency during cavitation depends much on the propeller and it is likely that a propeller designed for Volvo stern drive handles cavitation better than the one we see in the pictures. Also the bigger pitch of the stern drive propeller probably helps in this situation.

In order to get better thrust you need a bigger propeller. Since I don't know the target rpm, I can't recommend any pitch. There seems not to be room for a bigger diameter, thus you need more blade area. Maybe 4 blades as well.

Your current propeller looks like EAR~0.5. You need something like EAR=0.75 to get full thrust at 10 kn without too much cavitation.

[CDK]

Current shaft angle with the hull bottom is 32 degrees!

[Joakim]

Current shaft angle with the hull bottom is 32 degrees!

You can't measure the angle accurately from a picture not taken perpendicular. I have tried once and got an error of 15 degrees.

But yes the angle looks rather big. It is quite laborious to change the angle, thus I would first try another propeller.

[anthony goodson]

Good Morning
Where is this 80hp coming from ? Perkins Prima {Volvo MD22] or similar.

[CDK]

You can't measure the angle accurately from a picture not taken perpendicular. I have tried once and got an error of 15 degrees.

But yes the angle looks rather big. It is quite laborious to change the angle, thus I would first try another propeller.

Imported the jpg in CorelDraw, drew the angle, rotated it away from the perspective and measured it. Not exact I agree, but certainly not more than 2 degrees error. No waterline visible, so I had to take the bottom as a reference.

[rich99uk]

80hp is on FORD TRANSIT 2.5 engine
if CDK is right and the angle is that steep does that mean that would be my problem?
With regards my Revs i cant tell you much as i dont hav rev Counter
With a transit engine you get your HP at the higher end of the revs being 4000-4500

[baeckmo]

Well, rich99, w/o reliable tacho readings, there is nothing to hang your hat on. Quick check on propeller data tells that your engine:

1/ would not reach full power rpms with the setup
2/ if revving into the fourthousands, it would cavitate heavily

In neither case would it be able to propel those 2 tons at anything like planing speeds. The thrust line from that propeller is pushing her nose into the air and loosing lots of power because of bad inflow conditions (I think CDK is coming quite close with the shaft elevation angle).

Sorry, but you have to redo and do right. First, there is no Volvo drive with 2.0:1 gearing. Second, the aft pad on that hull tells me it has been designed for water jet (originally or as an aftermath). Third, with fresh 80 horses, and propped to reach correct rpms, you have a weight to power ratio of ~25 kg/hp. With everything correct that should bring you to something like 18 kn.

BUT NOT WITH ALL THAT SCRAP HANGING FROM THE BOTTOM! You have two options: either a correctly designed tunnel, or a correctly designed surface propeller. Both require correct numbers for gearing and correct propeller.

[anthony goodson]

Good morning Rich
May I make a further suggestion .from the work you have done so far you are obviously quite a handy chap and I think you would manage this quite easily. Remould the aft end of the keel to continue the profile to the transom ,easily done,screwed wooden former, GRP ,fair in and flowcoat.this will eradicate the low pressure area here. Fit an outdrive leg using the new lowered keel line as a datum for the anti cavitation/ventilation plate.This will give you ,with your NEW engine,120HP [less the losses from two sets of bevel gears] enough horsepower to plane this hull.I know this isn't what you want to hear but to my mind is the most commonsense approach

[Joakim]

It would be nice to know the real shaft angle, but if it is really that steep, wouldn't it push the stern up and thus bow down. Especially compared to horizontal shaft (e.g. stern drive) that certainly pushes the bow up. The rising bow further increases the angle of thrust compared to horizontal, thus making it worse at ~10 kn.

Why wouldn't a stern drive be an option? Due to "water yet" hull form? At 18 kn stern drive should be a very good option for a normal planing boat.

[fcfc]

If the engine is a NON turbo 2.5l Ford transit engine, the 80 hp may be a bit optimistic.
www.lancingmarine.com/fsd.pdf

[daiquiri]

I do not understand...
Ciao
Bit, we are both "students" in this case. ;)
Since it is an issue not related to this particular boat, I have opened a new thread about general theory and practice (actually, more about my doubts... ;) ) of inclined props here:
http://www.boatdesign.net/forums/props/propeller-performance-vs-shaft-inclination-30481.html#post322314
because I believe it deserves more consideration and would be off-topic to discuss it here.

[rich99uk]

Hi all.
I really wanted to get away from the outdrive leg because as i said it was always going wrong and a lot more maintinace than a shaft. But something worth thinking about Cheers.
It wouldnt be to hard for me to change the angle of the shaft as i have now aquired a v drive Borg warner Box.
what i am in the process of doing is welding up a cradle for the boat and i will bring it up on the scrubbing berth on a high tide. then i can take a few days doing the job. With the boat on the cradle i can cut all the keel off. And I have a V bracket I can put on to support the shaft. At the same time i will sort the rudder out as it wont need to be so Big. What you guys think ?
It is a non turbo. However i am in middle of converting it to a turbo. which involves New 130 psi Injectors different fuel pump so i can turn up the fueling turbo wich is a 3L iveco turbo (Turbo being bigger so it wont quick in untill higher Revs.) / Charge cooler and other bits. Lancing do a 140HP i think which is basicly a souped up ford engine.
Or Beacuse i have this New Borg Warner Box i can just put in alot bogger engine as i will not be limited to space.
I am having a problem identifying the ratio of the Box as half its plate is missing but i will open a new thread for that.
Just out of interest if the ratio of the box is less for example 1.25-1 or even 1-1 could i still use it and get good results ??

[Joakim]

If you are anyway putting more power and doing rather big modifications, you should definitely go back to stern drive. At 10-18 kn the difference is not that big, but with 140 hp you are going to be around 30 kn with good setup. At those speeds the difference between a good shaft setup and a standard stern drive is easily 5 kn.

A good stern drive should not need a lot of maintanance and should not cause any problems.

1.25:1 or 1:1 gearing is not going to work well with a 4000+ rpm engine. If the propeller shaft is rotating 4000+ rpm, you are not going to get a decent efficiency. At 3000 shaft rpm you can still get a somewhat reasonable efficiency around 30 kn, but you would be much better off around 2000-2500 rpm.

[rich99uk]

Ok so that kinda ration would be more suited for a Ford D engine which is about 120hp at 2500 revs ??

[CDK]

A good stern drive should not need a lot of maintanance and should not cause any problems.



What sort of stern drive should that be Joakim?
Certainly not the disassembled ones I see whenever I visit a marine and also not the 6 I've had.

[Joakim]

What sort of stern drive should that be Joakim?
Certainly not the disassembled ones I see whenever I visit a marine and also not the 6 I've had.

I (actually my father) have had only one sterndrive boat. It had a Mercruiser 165 from 1977. It served well about 10 years without much else than yearly oil change. Then there were corrosion problems with the power trim and also the hydraulic tubes were replaced. Soon after that the drive was driven aground so badly it was replaced with another used one which worked OK untill the boat was sold maybe five years ago. The raw water cooled engine needed much more care during those almost 30 years.

[CDK]

There is an interesting thread here "can a stern drive sink your boat".
Yes it can and it will if you keep your boat in the water long enough.

Once there were OMC drives without the dangerously unreliable rubber bellows, but they had other weaknesses built in to leave you without propulsion when you needed it most.