semi displacement

[whitepointer23]

Originally Posted by PAR

Assuming no slip, you'll manage about 12 MPH (10.2 knots) with 75 HP at that weight and I assumed 12 degrees of deadrise and a moderately draggy hull. You will not have any reserve, so a contrary current and a head wind will slow you down rapidly.

so if i went to 80 to 100 hp it should be a good setup. i may not ever do it but the way fuel is going it might be an option. the wife wants to keep the v8's, she loves listening to them.

[u4ea32]

White, I think you can do a decent estimate with your boat as is. Just run the boat at 10 knots and measure your fuel burn. Gas engines burn 1 gallon per hour to make 10 HP. If you can go 10 knots burning 7.5 gallons per hour, then you'll be able to go 10 knots with a single 75 HP diesel.

Obviously, the one diesel will be lighter than the two V8s, so do this test when as light as possible (empty tanks, no gear, no other people).

As you know from screwing around in a dinghy with an outboard, semi displacement speeds want the CG further forward than full planing speeds. So the decreased weight aft will help with performance -- e.g., your boat with a single diesel will perform far better at 10 knots (semi planing speeds) than your boat with the current twin V8s.

With my boat, which is a bit longer but about the same weight, I need about 120 HP to go 10 knots, 200 HP to go 120 knots. But my boat has a very aft CG (twin big block engines, notched transom) so it might perform a fair amount better with only one engine.

I'm in the same process as you. I've pulled the twins out, replaced the transom, built the engine beds for the single. Still a long way to go before I know it all works, but I'm not confident enough to remove that much HP and speed. I should be, but I'm not. So I'll still have 200+HP when done, once I decide on the engine...

[Brian@BNE]

Originally Posted by u4ea32

.........

With my boat, which is a bit longer but about the same weight, I need about 120 HP to go 10 knots, 200 HP to go 120 knots.

.....

Hmm, that's quite a rocket ship David!

[u4ea32]

Originally Posted by Brian@BNE

Hmm, that's quite a rocket ship David!

Um, I meant 20 knots... it only goes 55 knots flat out using 770 HP. The wind resistance starts to make a difference above around 40 knots.

[whitepointer23]

Originally Posted by whitepointer23

it is a planing hull, i will take pics next time i slip it.

finally took the pics

[Willallison]

Ok...
Adding 1 metre to the wl length will marginally increase your displacement speed.
A sudden sweep up towards the rear is unlikely to increase that speed. In fact if you try to exceed hullspeed (approx 6 knots) then in fact it will likely slow you down.
I agree with Paul... you won't get better than around 10 knots max with 75hp.

Almost certainly cheaper - and more succesful - to sell the boat and buy another....

[whitepointer23]

Originally Posted by Willallison

Ok...
Adding 1 metre to the wl length will marginally increase your displacement speed.
A sudden sweep up towards the rear is unlikely to increase that speed. In fact if you try to exceed hullspeed (approx 6 knots) then in fact it will likely slow you down.
I agree with Paul... you won't get better than around 10 knots max with 75hp.

Almost certainly cheaper - and more succesful - to sell the boat and buy another....

i agree will, when i was talking about turning the hull up i was thinking of the slight up turn that semi disp hulls have. the idea of adding a meter was because i want to build a large swim platform and thought why not follow the hull shape to get a bit higher hull speed. as far as selling this boat, i would not like to part with it.

[Willallison]

You will actually find on many semi-displacement hulls that whilst the buttocks do tend to sweep up as they run aft, they actually "turn down" again as they near the transom. This is called hook and is used in an effort to prevent excessive squatting. It has much the same effect as adding trim tabs. It is inadvisable in a planing hull as it can cause dynamic instability at higher speeds, though a number of not-so-fast planing boats have been known to use it to good effect.
If you were only going to run around at 10 knots, then you could in fact do this with the hull extension...
I know a guy who extended a 26' lancer that had a pair of chryslers in it... he made his about 32' long.... I've never seen it out on the water though, so I can't attest to the success or lack of it....

[whitepointer23]

i did have a 38 ft sambrailo semi displ. it was an old jarrah and oregon cray boat, it had a slight up turn with no hook. it cruised at 9.5 knots on the gps. i have seen a few boats with wedges fitted, i guess that would do the same as a hook.

[whitepointer23]

Originally Posted by Willallison

You will actually find on many semi-displacement hulls that whilst the buttocks do tend to sweep up as they run aft, they actually "turn down" again as they near the transom. This is called hook and is used in an effort to prevent excessive squatting. It has much the same effect as adding trim tabs. It is inadvisable in a planing hull as it can cause dynamic instability at higher speeds, though a number of not-so-fast planing boats have been known to use it to good effect.
If you were only going to run around at 10 knots, then you could in fact do this with the hull extension...
I know a guy who extended a 26' lancer that had a pair of chryslers in it... he made his about 32' long.... I've never seen it out on the water though, so I can't attest to the success or lack of it....

did he move the stern gear back as well, it would be strange having rudders and props 6 ft forward of the transom.

[whitepointer23]

Originally Posted by u4ea32

White, I think you can do a decent estimate with your boat as is. Just run the boat at 10 knots and measure your fuel burn. Gas engines burn 1 gallon per hour to make 10 HP. If you can go 10 knots burning 7.5 gallons per hour, then you'll be able to go 10 knots with a single 75 HP diesel.

Obviously, the one diesel will be lighter than the two V8s, so do this test when as light as possible (empty tanks, no gear, no other people).

As you know from screwing around in a dinghy with an outboard, semi displacement speeds want the CG further forward than full planing speeds. So the decreased weight aft will help with performance -- e.g., your boat with a single diesel will perform far better at 10 knots (semi planing speeds) than your boat with the current twin V8s.

With my boat, which is a bit longer but about the same weight, I need about 120 HP to go 10 knots, 200 HP to go 120 knots. But my boat has a very aft CG (twin big block engines, notched transom) so it might perform a fair amount better with only one engine.

I'm in the same process as you. I've pulled the twins out, replaced the transom, built the engine beds for the single. Still a long way to go before I know it all works, but I'm not confident enough to remove that much HP and speed. I should be, but I'm not. So I'll still have 200+HP when done, once I decide on the engine...

sorry for the late reply david, i am going to try your idea so i have a reference to work from. do you have photos of your rebuild , would love to see them.

[whitepointer23]

i have tried the boat now with both engines, i only had one running before. the results i got were 5 kn at 1100 rpm, 7.5 at 2000, 8.4 at 2400, 15 at 3000, 22 at 3800. 8.4 is the maximum non planing speed with out making huge wake, 14 knots is about the minimum to stay on the plane. i have been thinking the cheapest way to go to diesel is to fit 2 75 hp engines as i have one already and have found a second pretty cheap one. if i use the existing running gear it will be direct drive which is not ideal but i think having 2 props should compensate for the smaller prop size. the vic prop calculator shows this set up to have a top speed of 18 knots with 14x10 props, do you blokes think this is feasible or a bit far fetched. 18 knots sounds a lot for 150 horsepower. also has anyone tried using a standard exhaust manifold with copper pipe wrapped around it for cooling. and if i use the diesels i will have 2left hand props instead of counter rotating, will this matter . thanks in advance for everyones input.

[Adler]

Have you thought to modify the Propeller's Shaft angle at a range of 4° to 5° maximum and to fit lower Dia. Propellers ?

[Easy Rider]

To have a steeper quarter beam buttock line (angle) may be better than a hooked bottom. Hooks and wedges cause drag and many hooked bottom boats I've seen run bow down from excess hook. Higher QBBLs will only make the bow rise which I think is the more desirable of the two undesirables. I'm not sure a hooked bottom is ever really good at all speeds. Albin's 25 had a banana shaped bottom aft but to keep the bow down when they put a bigger engine in it they hooked the bottom and I personally prefer the banana stern. If a hooked bottom works well I suspect it is only at a certain speed. I also think a semi-disp hull should have curved sides at the chine. Straight sides parallel to the keel is for planing hulls only (I think). Just say'in.