how to get bow lift

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

my boat is a 27ft flecher with surface drive

cheers gerard

 

Realistically it is going to be in the Props, maybe prob height (or depth) as well. Good prop guys are going to eb able to dial in some bow lift for you, and be prepared to run props lower in water for more trim effect if not enough.

Otherwise look at weight distribution.

Warped trim tabs (Arneson) were produced that were meant to produce some bow lift - but don't know how effective or if they are available.

There are hull mods that might help - but too much work.

 

.... of course the other option is to go away from the surface drive and fit an outdrive!

 

but the outdrive wont handle the power of the yanmar 440

 

I see that you are still trying to cope with that problem. Can you post a few photos of the transom, seen from behind, above, sideways and whatever you can. Let's see what can be modified in a less painful way possible.

The sterndrive could actually be an option, if you can't find the original yanmar sterndrive for your power, you can try to contact one of independent manufacturers which could taylor a drive to your needs.

 

how about ballast.... cement will do, remember to weight it first and do some simple trim calculation to get the trim you want

 

if you flick over to boat design and scroll down to the bottom i have pics on there

p.s i really appreciate the help guys

 

I would need to see one more pic, that I can't see over there: one that shows the whole transom, from the sheer line to the bottom. Do you have something like that?

 

is these any good i can tale a few more tomorrow

 

As Adam said weight distribution could make a lot of difference, and could be the easiest to achieve. Trimming that prop may not be as simple. If the shaft length allows you could try a wedge plate on the transom to get the prop up. Weight back and prop up should make a difference. Otherwise you may want to consider building the bow up so it's physically higher, or stick to calm waters

 

Gerard, first try with adding the weight aft, to fix the CG. More aft you place it, less weight you'll have to use. The ideal would be inside, right behind the transom (if there is room).
Or, if there is some big weight near the bow, try to remove it or place it more aft.
If that doesn't work or can't be done for some reason, than go for the tabs.
http://www.arneson-industries.com/page.php?type=products&id=rocker

At the end everyone is telling you the same things, basicaly.

 

The first thing that I see as a problem is the fixed surface drive. It has a downward (negative) trim angle which is causing stern lift. This cannot be changed while the boat is running and this is the core of your problem.

A sub section of the drive is also the stern lift the rudder is generating which also compounds the problem. Its shape and rake is not enhancing any bow lift.

With the drive that is on the boat, you are physically limited in diameter of the propeller and thus the reduction ratio in the marine gear. Not a well balanced system.

I know you did not make the drive but it is what is causing your problems and your attempts should focus on this point first. This is evident by your many attempts to run a higher rake propeller (chopper) and the positive results of such.

You have not touched on the hull design itself, nor the lcg of the boat. These are areas that can also influence the boats attitude. What others are trying to tell you with the weight addition is the change the mechanical balance of the boat in an attempt to get the boat to rise in the front. Doing so will also make the propeller more submerged and load the engine more as its angle and height is fixed in relation to the transom angle. What goes up on one end goes down on the other. With the added weight and extra load you would naturally increase blade diameter and reduction ratio but your drive limits your choices here.

As the front is raised with the mechanical lcg change the rear lowers as a result. What this does is compounds the problem at hand. If the mechanical change works in getting the rear down and the front up, the shaft angle is also changed in proportion to the raise in the nose height. Thus as the negative angle increases the stern lift increases and the bow goes back down. Another negative effect of this is the boat will be slower if it does work. Why? Because the depth of the hull in the water increases as the boats attitude increases due to the mechanical attempts to change the lcg. Therefore the drag associated with the cure is increased and the speed will drop. (Imagine a large heavy cruiser that cannot get over the hump) The most obvious reason that the mechanical change is not good is down the obvious fact that you are going to add weight to the boat itself. You can do the math but you are looking at adding some 500 kg of weight to the stern to get the bow up.

Is this just a go fast boat or a work boat?

 

Fin on skeg is an option.

The Arneson folks suggested a small fin (hydrofoil rigged to provide down force) on a 120 MPH 18' Donzi Project. A similar fin on your rudder may work well to carry the bow.

http://www.arneson-industries.com/18'_donzi.htm

 

Does the boat have trim problems always, only at high speed, or only during acceleration?

 

You could try with Flexitab's, composite flexible tabs which can build rocker in your hull, effectively increasing running trim angle. They are hydraulically operated and you can find best trim at all speeds. Also, with a single surface drive, a pair of Flexitab's could improve significantly the handling of the boat. Visit www.flexitab.com or call +39 081 7901909