Help With Inboard Shaft Angle

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.

 

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.

 

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.

 

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.

 

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.

 

The boat is 21 ft

 

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.

 

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.

 

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.

 

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

 

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.

 

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?

 

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.

 

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 !!