Flat bottom planning hull speed

Does anyone know how to calculate the angle in which water raises after it passes under the hull and exists at the stern?
It must be a combination of weight (how far under water the trailing edge is pushed) and hull speed (the slower you go the quicker it kicks up back to natural water level).

 

Any small dyno that will go to 8,000 rpm and 20 hp will work.

The problem is that outboard motors dump their cooling water into the exhaust and dump the exhaust down near the output shaft. this means that you are going to need to make an adapter that will let you mount the motor so that it can drive the dyno (mount something to the prop shaft and have it drive the dyno) and then figure out how to capture the exhaust so that it doesn't spray water all over the place. This is why there aren't a lot of outboard motor dynos around.

Good luck.

 

You said earlier that the race officials test the engine? or determine that the engine is in fact 15 horsepower??
So technically they must dyno it to see what it is actually putting out

BUT, a big But, if they just look under the cowl to see that you have not turbo'd the unit, there are a couple of things that you can do to squeeze a bit out of it.

1) put in a molybdenum disulfide additive in the mix oil, about 2%,
2) have a machinist build a 1 to 2 inch extenstion between the back of the carb and the face that the carb was attached to that has the same diameter as the carb outlet throat.
The reason is that when you are WOT, you are pouring fuel into the cylinders very inefficiently, ie a very rich mixture of fuel per given volume of air, often the fuel will not completely evaporate from the venturi of the carb to the cylinder, ie un-evaporated fuel droplets end up in the cylinder, they do not denote but will burn slower than the vapor. Two things happen, there is fuel that cannot burn to add to the combustion force and secondly it burn oxygen without an significant increase in pressure, that the vaporized gas could use. The extended throttle tunnel length , will give more time for the fuel to vaporize and put a better explosive mixture into the carb, ie less droplets, more vapor

3) I used to own an industrial chrome plating company many many years ago, 1974. We used a flouridic based chrome formula that we could deposit 2 tens of a thousand of inch on a surface. I believe, and I hate to say the I forgot the number, that we could lower the coefficient of friction to half of aluminum to steel in a cylinder. This is a chrome coating, but satin coat, as chrome is usually deposited on steel, (in real life chrome goes on satin and then is polished to shine)
We put it on at about 4 amps per square inch, which is about twice the normal industrial chrome current density. This coating decreased the coefficient to a value close to teflon on steel, but the thin coating had a Rockwell C hardness of about 72, I think a file is about 70 RC
Anyway, we had a guy come into the shop with the components of a snowmobile engine, as he drag raced a 550cc class engine that had to be stock with external MODIFICATIONS, but there was not at the time any ruling for surface coatings. He came back a year later to have another engine done and told us that he pretty much owned the drag class for the previous year until others found out and began to play with the same idea.

This is very fine tuning, but it seemed to work for him
The process was called Dynalloy, and I do not know if anyone is still using this process but I suspect that any chrome shop could BURN their normal chrome solution at the 4 amps per square inch with similar results,

anyway more ideas to consider

 

Hi thanks for the info.

They dont dyno the engines they pull them apart after the race and hand it back to you in a box after measuring and checking a few dozen components

The additive is interesting. Our engines are pre mix at 50:1. Would this still work and what does it do?

Brad

 

molybdenum disulfide is a friction reducing additive, that will reduce the coefficient of friction between the rings and cylinder walls. ( friction not sealing)
I would put this into the gear case as well, with a little internet work, you may find a complete gear case oil that has a high content of MoS2 again, small things but thing that when added up can make a big difference on race day.
When helical gears interact, most of the load is transmitted from the gears rolling tooth on rolling tooth against each other, but there is some slippage as each tooth just begins to contact the next gear and when a tooth is just about leaving a tooth on the other gear. By reducing this sliding friction, you can reduce horsepower losses. Another way to do this is to drop the viscosity of the gear oil. Most engine manufactures may specify SAE 90, but this would be for a gear case that they want to have high long term reliability, and I doubt that you care about how long the gear case lasts.

The biggest advantage though is to match the prop to take all the whopping 15 hp that you have available

 

So I would find Molbdenum disulfide in the powder for add it to the oil at 2%. Then mix to fuel at 50:1.
Any information on race fuels and octane etc?

 

moly will be available in an additive with oil already, do not mix the powder at 2%, too much, for the oil additive, just 2% for the gears find the premixed product

 

Maybe I missed it, but why is the bottom flat fore-and-aft, instead of stepped? Very little in powerboat hull design has been more clearly proven in competition than the value of stepped bottoms.

 

Stepped by where and how much

I agree steps do work over a certain speed and I think over 30 knots they would probably work. But there are two really big questions that someone needs to answer.
1. Does it work on a hull that has no dead rise (flat hull)?
2. Where, how many, how high, how wide etc?
Brad

 

stepped hull, and flat bottoms

A stepped hull would work on a flat bottom boat. Remember that the pressure is highest about a third of the way back from the wetted surface and from that high pressure line, called a stagnation line, the pressure diminishes to atmospheric to the transom. When you put in a step, then you have two high pressure stagnation lines. Assume only one step, the front half then has this high pressure stagnation lines, and the pressure goes from this high to the first step, to atmospheric pressure, then the step is ventilated to atmosphere. Then the water hits the hull portion behind the first step and another high pressure line results which then drops to zero etc.

The most important part to remember is that behind the first step, if there is only one, that this first "transom" so to speak is ventilated to atmosphere.

 

Flat bottom steps

Great info thank you.

Therefore i need help with dimensions, length, angle etc.
How do I make it ventillated??
Would 20-30-50mm work in step height. Is it perpendicular to the centreline?

I really need somewhere to start.

The planning length is atoms 9 foot. Therefore 3 foot from the front of he planning length would be the right distance back

help.

 

This I cannot help with as I have seen most angled back from the keel to the chine and some that even were reversed, ie angled up toward the front.
I am not sure, spacing or the number? again we have never built a stepped hull but the concept is to get a higher number of stagnation lines. I would imagine that you can pull some details off the net and look at the bottoms of some of the "go fast" that use this design. I would not want to offer an opinion of dimensions etc without first hand experience. Regarding of how you ventilate the step, again a quick look at some hulls, even in the boating magazines, you will be able to see that the short step comes through to the side of the hull side so that air can ventilate the back of the step, this is done to ensure that the pressure at the back of the transom is zero psiag, ie atmospheric, if there was water behind this transom, then there would be more drag.

 

You can ventilate the step with a tube though the hull right behind the step but its not needed, your high enough out of the water. judging by your video, a 35 mm step about 5.25-5.75 ft foward of the transom. 1 ft wide transverse section, then 30 degrees foward. Smoothe out the bottom also, t looks rough in that photo. fair, wetsand, wetsand, wetsand, paint, wetsand, clearcoat, wetsand.

 

What are you basing this on? Is this a guess? It is going to cost me money to try this out. I just want to make sure you know what you are talking about.
Thoughts....

 

I am basing it on your video. Without knowing all of the details of your boat, then yes it is a guess. It may not be the best solution, but it will provide some benefit. A slight downangle to the forward running surface will also help. Racing boats is all about setup and testing.