Theory of Hull Speed
 

Do Planning hulls have a hull speed where drag becomes so great that even with additional horsepower your boat will not go much faster. Or it requires alot more horsepower to go a little faster?

My new motor is faster with one person and little gas on board. When I load the boat up with 50 gallons gas full water 4 people and 6 water ski's on a 110 degree day the new motor will plane faster but at wot my top end speed is the same as my old motor.

I figure with the additional weight I sit low in the water and I just have too much drag at wot that the additional hp is not noticable at speed. The additional hp is noticable out of the hole and comming on plane.

The other thing that might be draging me down is a 4 blade prop. Maybe I will go back to a three blade?

Is hull speed a brick wall or should I keep trying? Maybe trim tabs will give me more speed with a load?

Thanks

Tell us about your boat. Engine, drive,HP, what parts in engine. GO INTO FINE DETAIL.

Uh-Oh; I'm not liking the sound of this! Check out the "Prop/Stern Lift" thread and get back with more info. like Tinkerboater said. --- Jer

Sounds like you've got a smaller boat. They are affected more by increased weight.

Also sounds like you torque curve on the engine changed form the previous engine. Since you're planing faster but not really gaining on the top end, you've added torque in the low and midrange, but not in the upper rpms. Thus, no real speed increases.

Just some guesses with the limited info.

I like your thinking Griff.

Tinkerboat,

Chaparral 1935ss short and beamy. Stock motor 350 2 barrel with an alpha i gen2 drive.

new motor vortec 355 stock cam, edelbrock 600cfm 4 barrel, edelbrock air gap intake.

Griff,

Yes this is a small boat 19' 3". I think the drag builds with the additional weight.

This was my experience: When I was testing my engine: One person light on fuel the motor would rev past 5k just like it will rev past 1k. I was thinking about repropping the boat.

With a full load and people in the back seats the most I could get was 4800 rpm. So my old prop is fine.

My old motor would also max at 4800. My new motor seems to have alot more hp comming out of the hole. Running empty the boat is faster. When I load it up It runs like my old engine at wot. I figure at about 46 knots fully loaded the drag on the chapparal hull becomes so great the boat is hitting the designed hull speed.

I was wondering what the drag curve looks like?

Thanks

I think what you have is fairly common. I had a 20' Crownline a few boats back, and I would lose 3-5 mph if I had 6+ people in it, compared to having just 2 and light fuel. It was the same type of boatas yours. Short and wide(8'6) beam.

So, Yes, I think your drag is greatly increasing with the weight added. All boats will have increased drag with more weight. When you start with a 3000# boat and add 1000#in fuel,gear and people, thats a big % increase in weight.

This is a favourite subject. There are three components to hull drag; displacement, wetted surface friction and turbulence as the water sucks in behind the transom. Planing hulls minimize all three compared to a displacement hull (sailboat). The math says that power should go up as the cube of speed, but in real life in a planing hull, power seems to go up as the square of speed. Mercury's empirical equation based on the results of many boat tests is a squared function. As a result the hull drag curve is a parabola.

The power available from the engine at different RPM is the dyno curve. Making some simple assumptions about how much of the dyno HP makes it through the drive to the prop, and how much of that propshaft HP is converted into thrust, you can plot the available prop HP curve on the same graph as the hull drag curve. Maximum speed occurs where the curves cross.

When this kind of model is calibrated with accurate test data for a particular hull, the predictions of speed vs. power are pretty accurate.

yeah what Tomcat said;)

Boat Speeds
 

Predicting performance powerboat speed and stabilty is somewhat more complex than it was 25 years ago. While the water (hydrodynamic) lift and drag are a function of profile drag, friction drag and at slow speeds, maybe some bouyancy; there is also a significant component of aerodynamic lift and drag to most well-designed, modern-day performance powerboats. As the speed increases the contribution of lower unit drag, cockpit drag and motor drag become significant also.

The lift and drag generated due to the 'planing' of planing pads or sponson bottoms on the water surface is a function of five (5) factors:
(a) Velocity
(b) Lifting surface area
(c) Angle-of-attack
(d) Aspect ratio
(e) Surface finish

I've just published a series of articles on powerboat lift, drag and performance in my monthly newsletter. You can get the back issues of TBPNews newsletter.

If you like, you can send me some details of your boat, and I'll run it through our design performance analysis software for you.

Also, check out the downloads in the on the AeroMarine Research website.

If you're serious about the real contributions design features make to performance and stability my book has all the details and formulae for everything...check out our publications at:
"Secrets of Tunnel Boat Design"
"Power Boat Design Software"
"History of Tunnel Boat Design"

/Jimboat, AeroMarine Research

Sounds like the cam and carb are too small to me. What are the specs on the cam? Duration at .50, lift,lobe seperation. Is this carb a vac. secondary?

Removed by me to a separate post.

TZ

Here's an example of a power vs. speed graph with hull drag and engine HP both expressed in prop HP. Hull drag in this case includes all sources of drag mentioned by Jimboat and is based on observations and test results. This is a good method for predicting what your boat might do with more power when you do not alter the sources of drag in any way (like when you raise the drive). Predicting what a different hull design might do requires Jimboat's more detailed theoretical approach.

I compared two engines on the same hull, light and heavy. This is not exactly your specs but you can see how the more powerful blue engine is faster when the boat is light (yellow) but when the boat is heavy (green) both engine curves (red and blue) cross the green hull drag curve at the same point.

Better breathing normally aspirated engines generally need to rev higher to reach their HP potential. You have to decide what kind of load you normally carry and prop for that load.

Just in case you're wondering, this is what would happen if you supercharge. And yes, you do need a supercharger.

This is my theory for my boat. I have hit a wall. When I load the hull up with all the weight the drag on the hull is my limiting factor. If I could double my hp without increasing weight I would only get a modest increase in speed. A cat hull design would solve the problem.

If this is true there is no need to try some new props. I tried a couple of different props a few years ago and had very mixed results. Running empty I could gain alot of speed by increasing the pitch on the prop. Unfortunally my deep v runs too deep from the weight in the boat the drag is holding the boat back.

I look forward to trying some new props but i will wait until I damage the one on the boat before I buy a new one.

If my theory is right longer is better even for a planning hull.

Thanks for saving me money on trying new props.