New boat problems

IMHO - try moving the weight and the driver back to just behind midship, allow the bow to rise up a bit, and this will give your two outside sponson a better chance of getting on a plane. I think your pushing to much water to get over the "hump" and onto a plane.

Hope you work it out, I give you full marks for trying something different!

All the best,

 

nice posts rick as per, yes the man has a funnel hull, its a real shame feller, but hey one learns along the way Even if you make her go fast she is going to land very very heavily um,

 

I did some wave analysis on the hull. At around 6 knots with level trim and 8" draft you should find the boat is trying to carve a trench right at the outboard transom something like 460mm deep (17"). That is what you are up against. The actual power required to overcome this and get on the plane is only around 20kW. Problem is you will never deliver this with the prop ventilating.

The only way I can see to overcome this is to dramatically alter the trim to bow up so you get the concave section above water level. Once on the plane the hole moves behind the boat and you get a water hump at the motor. Hence you should be able to set the motor quite high to operate on the plane.

You could also try to set the motor tilt so the prop is further from the transom. This will also help lift the bow.

You have the power to push the boat with a highly exaggerated bow up attitude while you are getting on the plane. Once on the plane speed will pick up easily. However if the bow dips into the water you will slow down but you should still keep on the plane. If you lose the plane you will ventilate until you can force the stern down and bow back up.

As noted earlier this boat is LIKELY to behave in a nasty fashion. I have never seen such a small boat able to produce a wave that will be over 800mm (almost 3ft) from trough to crest. That is the difference between the trough and the crest trailing behind the boat. It exists throughout the speed range with level trim.

It would be useful if you could provide some photos of the wake when the ventilation occurs just to confirm my analysis. It might also be something you could boast about. I expect you will challenge the local tug fleet for size of wake.

Your proposed foil will be an airfoil in the normal trim and do nothing. If you do manage to get the stern down to get the foil and prop in water, the foil will tend to lift the stern and force the bow down so you get the opposite of what you need. And no setting it the other way adds downward force and you will get more drag. If you tried hard enough you could bury the stern.

So adjust the motor such that the cavitation plate is level with the hulls, tilt the motor to probably the last notch, shift as much weight aft as you can without swamping the stern of course, don a life jacket and push the throttle slowly forward. If you manage to get on the plane treat the controls gently.

This is only intended as a temporary solution (if it works) until you can correct the concavity and other defects in the hull shape.

Rick W.

 

OK thanks, I might have to make a wedge to fit under the outboard bracket to get her going! I will try this on the weekend ---
PS, I did copy & roughly scale the design from a site found on the net but after 3 days of searching I cant find seem to find it anymore - wonder why!

 

Well I think that I have to except that the bow shape has to be changed.
I think I might be able to take a mold off the boats inner hulls & then make a couple of foam filled - glassed over infill pieces.
I plan to dry the boat out on the beach & jack up the bow, wax up the area & take a glass fibre template. The I can work on the infill at home, once made up I can dry the boat out again & glue & glass them in place.
My question is - shall I make the bow symmetrical or is there a better shape for the infill pieces? Do I need to incorporate a spray rail?

 

The main thing is to get rid of the concave portion. Symmetry is not crucial. You could get a better ride if you extend the bow of both lower hulls forward and upwards to get something more like the hulls linked in post #10 on this thread. They would then protrude forward of the existing hull. Maybe this is for the winter program.

Did you try to get it on the plane with weight aft - just curious for my own education.

Rick W.

 

Ok Rick I have found that site http://www.johnsboatstuff.com/Boat Designs/TRIdent 12/wp-10.htm
Yes I got her on the plane but the boat was very unstable & non responsive with erratic steering!
I agree about the winter work & in fact looking falward to her next set of improvements. I built a 20' cat a few years ago & have saved the bow section molds, I might be able to use these somehow?

 

I looked at the waves with the concave section removed. There is still a point between 6 to 7kts where there is a hole where the prop is. It is not as deep but it is still there. It is behind the motor at 10kts.

The hull floats are quite short relative to where the driver sits - does it float bow down? If it is doing this then it will tend to produce a larger hole at the outboard.

The advantage of extending the hulls forward is that you also get more buoyancy forward and this helps get the bow up.

Also by having a long tapering section at the bow you get lift up front. Your bows are more like a displacement hull than a planing hull. You want a shape more like a ski at the front than a wedge.

Rick W.

 

Learning all the time! so I might be waisting my time making infills & might as well get her back home for more serious work?
Here is my first cat bow shape, like I say I have the mould.

 

Have you tried that with an outboard? The shape looks superior to what you have with the modified aluminium boat.

The trident hull is somewhat different to what you have built. I still wonder how it performs.

Rick W.

 

Well the little boat is coming out of the water next Tuesday, I have arranged for her to be taken to a local storage yard where I can carry out the work. I think I will go the whole hog & extend the hulls, I will post some pics when she is on dry land.
I found this pic, the prop looks very shallow?
http://www.breezecraft.co.nz/profile.html

 

Here's what you might do.
Mount a very narrow nacelle between the hulls, with a blunt transom end a couple feet forward of the OB lower unit. The nacelle will act as a mini hull and make a stern wave that will shoot solid green water at the OB prop instead of aerated water. I have seen some commercially made single outboard powercats in the US with this design, seems to work. The concept has some commonality to a tunnel on a flats boat, so likewise the exact dimensions of the nacelle are probably fairly critical.

 

More delays as the hiab (mobile Crane) cant transport my boat until friday
Bwd, yes, I would of thought that might work too?

 

Here is a mail i have received from Breezcraft -
Hi Martyn,

Sorry for the delay in replying, I have not been at my desk much as I am sick.

Speed with a single 50hp outboard is 25 - 30 knots. We fit the single outboards to one side as there is less drag, and we have found that 40hp is max size and usually a lightweight model. We get better performance from a 40hp one one side than a 50hp in the middle as there is less drag in the tunnel.

Speed with twin 25hp outboards is 30 knots plus, these are the better option.

We can supply the boat in open form, I have attached a pic of a model we made like this, you do not have to have the longitudinal seat. I have also attached another pic of a hull with a cut down cuddy.

We would supply a twin engine bracket which could take a single to one side or twins.

There is not much else I can tell you as most of the info is on the website.

This boat is a great little runabout, very stable and plenty of room for its size, it has been a popular model.

Regards
Sharon

 

The underwater shape of these boats look distinctly different to the ones in the referenced site showing the so-called wave piecing bow. These look almost conventional bow for a planing cat.

Also note the dead rise on the hulls rather than being flat.

Clearly they have a similar problem as you encountered because they are using the offset outboard. Would mean constant helm to hold a straight course but not much loss in performance.

I think you would get best results from a bow like the Thundercats and the stern with some deadrise like that in the Breezecraft.

You did not give a lot of information on the testing you did with yours at speed. I would be interested in more detail on what happened.

Rick W.