how to give a hull bow lift

[weehenry]

has anyone ever added anything to a hull to get bow lift

my boat is a 27ft fletcher with surface drive

cheers gerry

[tom28571]

This issue doesn't come up very often. It's a bow too high that is more often the problem. After the boat is built and weights are in place, there is not a lot that will do the trick. Some designers purposely put in some aft rocker in the bottom which draws the stern down and the bow up. This is effective on a planing boat but does rob a bit of efficiency.

[weehenry]

my problem is that when the when i go over the 50mph mark the bow starts to hook in and makes the steering a nightmare
witch i guess is killing a lot of my speed

thanks for your reply

gerard

[daiquiri]

Originally Posted by weehenry

my problem is that when the when i go over the 50mph mark the bow starts to hook in and makes the steering a nightmare
witch i guess is killing a lot of my speed
thanks for your reply
gerard

A bow-down condition can be generated by either one (or a combination) of these:
- center of gravity too forward
- bad surface drive trim
- keel line not straight (keel with a rocker)
- a transom with a wedge (hook) or a similar device which modify waterflow exit angle
- if it is a stepped hull, could be due to badly positioned steps
It can be due to many things, as you can see...
Do you have a photo or a drawing of the underwater hull form?

[tom28571]

Based on your reply, I'd look at the CG being too far forward. Perhaps, when you get that fast (50mph) there is so little hull in the water that the weight forward causes the bow to fall. Normally, you would tilt the lower unit forward on an outboard but, I don't know if that is doable on a surface drive.

[Design_1]

Wee-
A good start would be to place a straight edge, parallel to the keel, along that transom/running surface edge to see if there is any hook or wedge applied to the boat already. Start at the chine and work your way down to the keel. Most hook or wedge will be applied within the last 4 inches of the boat, but measure 8 to 12" onto the running surfaces. If there is hook or wedge already applied, it can be sanded down to raise the bow and reduce bow steering.

Hope this helps. If you need more definition of how to measure the hooks; I can send you some photos. I have a hull upside down in the shop now.

Regards,
Chris

[weehenry]

thankyou so much for all your replys i really appreciate the help
Last year i removed the super structure and replaced it weh a 2mm aluminum flat deck and aluminium floor. i completely gutted the cabin and replaced all the waterlogged bulkheads with foam bulkheads i havent even got a floor in the cabin. i fitted a 440 yanmar with 2 speed transmission to a fixed lancingmarine surfacerive at 4 deg. i have the engine tighe to the stern with the coupling almost rubbing the stern. fuel tanks witch only hold 8 galon each are placed tight to the stern along with the battery maximum speed is 65 in a good chop but only 57-58 in calm water . i have checked the hull for hook and it was straight maby too straight maby lackin in some rocker . i wonder is it possible to cut that whole section where the engine sits and lift it a few inches that might letthe bow up

i have tried to find a chopper propeller over 16inches in dia but very hard to get id say that would make a difference you know the the old rolla round ear i think it was called a rolla record there is a pair on ebay at the minute but they are only 21 pitch and im running a 25 cleaver at present

[weehenry]

[weehenry]

[tom28571]

Considering what you have already done, it should not be too hard to introduce a bit of rocker to this stern. Mold an extension to the existing hull bottom with a bit of rocker. How much and how far? experiment starting with maybe a 8" extension that curves up 3/8 to 1/2". Make the rocker tangent to the hull bottom at the transom so that there is no sharp break there.

I'm really guessing a little here, but that is what some builders do for the same objective although they build it into the original hull.

[weehenry]

[weehenry]

is that what arneson are trying to do with their rocker tabs

thanks for your quick reply

gerard

[Guest625101138]

Without a cabin or windscreen the hull is set up reasonably well from an aerodynamic perspective to fly downwards by the bow. This would result in significant force at 50mph.

Is the deck in its final configuration? If not then think hard about underwater modifications before you set the deck up properly.

As an example I used to have a deep keel boat that got an annoying roll when running beam to wind. The hull would lay over rather than sit square. The only way to beat it was to adjust the weight or some boats used trim tabs to lift one side. It was an aerodynamic force not hydrodynamic causing the roll. Surprisingly it rolled into the wind not away.

The attached exaggerates the point because it is only 2D and there is no water surface. The surface provides ground effect. But you can see there is likely to be a lot of net downward pressure particularly as the water increases lift due to speed.

You could test this by fitting an aerofoil on foredeck set up to give lift. An alternative would be an air dam on the stern. The latter might be better for aerodynamic stability.

The point I am making is that over something like 40mph you really need to be thinking about the aerodynamics not just the water interface.

Rick W

[daiquiri]

Originally Posted by weehenry

Last year i removed the super structure and replaced it weh a 2mm aluminum flat deck and aluminium floor. i completely gutted the cabin and replaced all the waterlogged bulkheads with foam bulkheads i havent even got a floor in the cabin.

Is this what the original boat looked like? It is the only pic of a Fletcher 27ft I've found around:
http://www.baadgalleri.dk/html/gal_visbil.asp?ID=1744

This superstructure's CG looks pretty backward. If that is the case, then its removal has saved you a lots of weight but has probably moved the CG of the boat forward. So now the stern, being relieved from the weight of the superstructure, trims up.
With the CG fwd, your bow's contribution to the flotation becomes more important. Now, since the bow is curved in the forward part you basicaly do have a keel with a rocker now. I wouldn't be surprized that at high speeds a suction force is created in this zone, acting downwards. It is a known hydrodynamic phenomenon and the steering problems you've described fit that picture perfectly.

Rick's aerodynamic considerations are also valid, so you have a combination of these effects, imho.

What can you do? Well, I see that you enjoy messing with these things, so it shouldn't be difficult for you to try one of these (in order of difficulty):
1) trim the surface drive a few degrees up, if that is possible.
2) try to move the CG backwards by placing a well-fixed ballast somewhere close to the transom. How much of it? I don't know - you have to experiment. But even just a few degrees of pitch-up trim can change your life radicaly.
3) add a fixed (but adjustable) trim plates (tabs) at the bottom of the transom. You'll have to make few runs to find their ideal angle.
4) One of Rick's solutions
5) the most difficult one: modify the trailing edge of the hull by giving it a slightly upwards-sloped shape. That should be the last resource, imho.

[Guest625101138]

The steadily rising deckline for the linked hull reduces down force a little. I expect the change would offset at least the weight of the cabin.

Rick W