Can You Help Me With The Shape Of My Hull

Hi Everyone

Ok attached are some pictures of the hull off my boat.
Now can anyone tell me is this a plaining hull or displacment hull or semi displacment or any other design ??

Ok a bit of history behind this boat it was designed with inboard it originally had a huge splash well for two or one outdrives up to 250 HP
When i got the boat i took the splash well out built up transom put 2.5 ford desiel transit engine in it with a shaft and i made the keel and and rudder. The engine is rated at 80hp a bit under power i know.
At the moment the bow comes up in the air and i only do 8 knots i dont seem to get on plane. I also have a faire size wake if that has anything to do with it.
when doing 8 knots the stern has no water at back. The water is flush with bottom of the boat !!

Is my Hull a planning hull ?
it looks like a fast plaining hull to me lol please give me your opinions.
I am in process of getting prop re-done but would appreciate any ideas

 

Yes, it is/was a planing hull. How big is it, what is it made of and how much does it weigh? 80hp apparently isn't enough to make it plane. The keel may be giving you problems too.

 

It is 21ft and weighs just under 2 ton I think.
before i put the shaft through i tried it with a volvo 270 leg and i got 13 Knots that was without Keel. It didnt plane it went through the waves front heavy didnt come up like it does now. With the leg it seemed to be front heavy
I Put shaft instead because it is lower maintinance leg was always going wrong and expensive. at the same time i rebuilt the cabin making it lighter.

 

It is made of fibre class

 

It has planing strakes so was intended to plane.

A good planing hull weighing 2t should easily get 20kts with 80HP. The shaft angle looks severe but tolerable. The keel adds extra drag but not a huge disadvantage at moderate speed. The centre "V" is quite severe but this should be compensated by the flatter outer sections.

The pitch of the prop looks small. Is the engine reving easily and going to maximum revs? What is the prop pitch? What is the prop rpm at full engine revs? What is the prop diameter? Are you confident it only weighs 2t?

It sounds like the Volvo leg was angled in too far such that the thrust was actually lifting the stern and ploughing the bow in. I expect the deep bow would make it skittish in a following sea even if the trim was correct.

Rick W.

 

Thnak very much for the replies
The Prop was 16 x 14 on a 2:1 ratio gearbox and i was not really getting full revs.at this point the prop does need balancing and sharpening ect.
The engine goes up to 4000 revs.
I have taken prop to Prop specialisted and they looked at pics done a few calculations and recommended the pitch be taken down to 11.2 ?

 

I have done some analysis on your boat and prop. It does not make sense with the estimated weight.

The attached screen dump is taken from the analysis using JavaProp. I have assumed the prop is getting 55kW (73HP) allowing for auxiliaries and gearing losses.

The thrust that JavaProp has determined is much higher than I would expect. My rule-of-thumb for drag on a planing hull is weight/8. This would be 2450N for a 2t boat compared with JavaProp 5486N. I would expect drag to be a bit higher as you lift to the plane but this seems way high.

You can see that the prop is not very efficient under these conditions - only 40%. So you should be able to do a lot better. The prop is working at relatively high slip. About half the blade would be stalled out if the shaft was horizontal. It will be worse than this with the steep shaft angle. Sharpening the leading edge will actually make it worse. You would be better off rounding the leading edge to improve the flow at high angle of attack so it is less prone to stall. Think of air flow over a wing near the stall condition. A curved nose helps to ensure streamline flow over the leading edge.

If we say it is going to take 5500N to get on the plane then this suggests the boat weighs more like 4t.

If this is the case then I believe your prop is simply too small. For an analogy, you have a prop suited to a large sedan when in fact you need a prop suited to a 4t truck so to speak. The prop does not have enough grip in the water. Reducing the pitch will not help because you do not have the grip in the water. I determined that the 11.2" inch prop will go a bit slower.

If you can fit a 24 x 23 prop and alter the gearing to 4:1 then you should be able to get around 13kts with a drag of 5500N. The prop efficiency gets up to 62%. It might even do better if the drag drops off a little as you get closer to planing.

So the first thing I would do is to confirm the weight. A 21ft planing boat should be around 2t but you never know. My numbers indicate that a smaller pitrch will be worse if the drag is as high as your performance numbers suggest.

An alternative to weighing is to do a similar check on the old engine. Do you have any details on the Volvo 270. What was the bottom gear ratio (there are three ratios)? What size prop? What engine rating? This would allow a data comparison.

If the boat only weighs 2t then there is a possibility that there is too much weight forward and you are not getting enough lift at the bow. It looks to be a narrow deep entry.

It is possible to get the motor weight further aft and improve the shaft angle by using a "V" drive box. However this is major surgery. I guess changing the gear ratio and using a larger diameter prop is not trivial either.

Do you know how other boats of the same design perform?

Rick W.

 

I estimate from the photo that the prop shaft is angled at 17 degrees. This is severe.

The vibration you want to correct may not have anything to do with static balance of the prop. You should be aware that inclined shafts induce vibration in a prop. The angle of attack of the blade on the upgoing side will be much less than the AoA when it is on the downgoing side. The situation is really bad if you have a two bladed shaft with high aspects blades. It induces high bending stresses in the shaft each revolution. The vibration gets better with 3 low aspects blades but could still be apparent.

Going to a bigger prop is likely to increase the level of vibration.

I also thought of another possibility how the prop could be spinning at 2000rpm while doing 8kts without using the full power. It could be ventillating. If the stern is at the water level then air might be getting into the prop and reduce its effectiveness. However I expect you should have some idea if the motor is not working hard. You could check fuel consumption over a cruise if it is not apparent.

Rick W.

 

I have just picked up my prop and they told me it was not balanced at all it was very bad.
The boat definatley weighs 2 Ton
However it might be worth mention my stern has got all the weight at the very back i got the fuel tank that isnt small.
and then the engine is in the lowest part of the boat. just behind it.
If you look at the pics on the side of the boat it gets deep just after half way down that is were my engine is. It acts as the piviting point so to speak.
I am wondering if the combination of cavitation due to prop and being stern heavy i getting stuck on my own bow wave. as i do get a large wake.
when i had the leg on it i had a 16x13 prop with 2-1 ratio gearbox and same engine. The only other major thing i changed is i have moved al lot of the weight to stern.
Since i have had the shaft in it. It even sits slightly higher at the bow, which is better as it was to heavy at bow before. Ill add another pic of it in water

The problem i got is that i havent got enough room to put a prop larger than 18 inch diamiter on.
The boat is on the scrubbing pad sat to have this prop put back on so i will measure the angle of the shaft then. I am also going to try moving some of the weight round as im going along see what happens.

 

Regarding Weight - Is the 2t with or without fuel and other provisions. How much weight when you are actually trying to plane.

I found information on the Volvo 270 leg that indicates it comes in three gear ratios. None is 2:1. I have done the numbers for the nearest to 2:1 i.e. 1.89:1 at 13kts with the 16 x 13 prop. The drag comes out at 5164N. Efficiency is up at 60%.

Still a huge amount of drag for a 2t boat and is consistent with 5500N at the drag hump. It would be interesting to take a closer look at the hull shape in displacement mode. You cannot avoid the wave drag of the displacement mode before you get on the plane. Playing around with trim can help though.

The drag should reduce once you get fully on the plane but getting over the hump is the issue.

The forces on the inclined prop when pushing are trying to align it with the flow so it will have a lot of lift with the steep angle. Like I said earlier it may be getting too close to the surface and actually ventillating. Fuel consumption should tell you if you are actually delivering the full power.

The experience so far has demonstrated that with the same power, almost the same prop but using an inclined shaft has halved the speed.

Rick W.

 

Can we clarify what revs the engine is running at at full throttle. I ask this because we - see explorermarine.co.uk - built a 7 metre jetrib with a STEYR 164 hp diesel, but due to the tuning of the waterjet impellor, the revs were held down from 4000 to 3150 which reduces the flywheel horsepower from 164 to about 120.
If the same is happening on your 80 hp Transit engine you could be down to 60 or less horsepower which would explain the problem.