Increased Hull resistance?

Hello all & thank you for reading my question. I am new to Hi performance boating and have a 20 year background in automotive motorsports.

I purchased a used boat that was rated by the manufacture (name withheld) at 65 MPH at 4600 RPM's with stock power and a 14 1/2 X 23 stainless prop. The boat came to me with a modified engine (454 with small blower), which I further tuned, and estimate the HP at around 550. I increased prop to 14 1/2 X 28 and can turn 5000 RPM now @ 70 MPH.

I feel this is not a significant increase in MPH with the power going to the bigger prop & increase RPM. As the 4th owner I found the hull's bottom fairly scared up at the rear 1/2 of the hull! It is not smooth and parts of it even look like a washboard Could this be where I am loosing my speed due to the increase in hydrodynamic drag? Could the loss be that significant due to the rough bottom? With the original numbers corrected the new speed should be around 86 MPH by formula that originally gave a 7.87% prop slip. I would be happy with 80 MPH that would give a 14.3% prop slip but the actual slippage is 25%. Does this seem unrealistic or could I really be loosing that much in the hull?

Thank you for any answers!

 

It is hurting your speed a bit, but I'm not sure if it's that much.

 

Which way does the washboard run? Do you have an idea what caused it?

Why not try filling and fairing the distortions or grooves with some premixed filler or epoxy mixed with medium density filler and then sanding smooth with a longboard? Sanding isn't the most fun thing to do, but your hull should be smooth. It will get a few extra mph at least.

 

At 25mph, a propeller slip of 25% would be OK but at your speed, it is much too high. Your prop is probably cavitating (not ventilating) which will quickly destroy it by eating the forward surface away. For sure, you don't have the proper propeller on it. I highly recommend that you buy Dave Gerr's book on propeller design and use. It should help you understand what is going on and how to approach the solution.

At the speed you are running, the drag is all frictional so any roughness on the hull planing surface is very harmful. However, the shape of the bottom is even more important at these speeds and small areas that are not fair or symetrical as the designer intended can cause real problems in stability and make the boat real skittish. I've no desire to go that fast and so have little first hand experience with the problems. I did get slammed about badly in a fast boat last year that would slide in a turn and then grab with such force that it threw me across the boat. I would have gone over the side at about 65 mph if a guy who knew this might happen had not held me in.

 

Guest, I plan to fill in & straighten the bottom & it does not appear that there was ever a hook designed into the bottom.

tom28571, this is the 3rd prop I've had on, 1st was aluminum 21 which seemed to slip bad out of the hole and cavitated even worse when trying to accelerate from a steady speed. The second was stainless 23 that came out of the hole better but still seemed to slip and also cavitated from a steady speed when accelerating hard. The 3rd prop a 14 1/2 X 28 with a slight cup, it seems to hook up and plane the boat maybe a bit slow & RPM does not exceed 4000 RPM when starting from a dead stop. In addition it seems to accelerate very quickly from 3500 RPM to the 4500 RPM range and continues to climb until 5000 RPM. It does not feel over or under propped. But the portion of the hull that is in the water at speed is the most damaged. I have no idea what happened to damage the hulls gelcoat as I am the 4th owner now & knew that I had some work to do to repair it, I just had no idea that this might be THAT bad. I am well versed in fiberglass and have made plugs and molds in the past as well as finished pieces so it is not beyond me to do any repair to this. I am really curious to hear if I could loose 10 to 15 MPH due to really bad bottom damage.

Thanks Charlie

 

It's hard to say without seeing a picture of how bad the damage is, but I doubt it would be that much. Unless it's really bad. Do you have a digital camera?

 

Guest, I could take photos but they wouldn't do any justice from that camera as the hull is white and you really don't get the depth of the washboard effect or the scratches or as what I want to jokingly call the non-skid bottom. I have emailed the factory and with their numbers the prop should be at 10% slip at the speed they say it will do. When I adjust the numbers for my setup the 10% should yield 58 MPH at 3500 RPM. They did say the max speed was between 55 & 58 MPH with a 21 pitch prop on it. So based on what they have said the only thing is the hull that would effect the speed that much. Could this be reaching terminal hull velocity? I see many other with similar hulls that can run in the 80 MPH range.

 

Your boat bottom is definetly a big problem. even small irregularities make a huge difference in the way a boat handles especially at speeds in excess of 60mph. I have a boat with an exceptionaly clean and straight bottom. A couple irregularities maybe the thickness if a matchbook cover in a section of about 1 inch over 3 feet was enough to gain me 5mph at 70 once straightened. With higher speed it is even more magnified. Try this, stick your arm out the window of your car at 30 and again at 70. Now imagine the difference with the density of water. Don't listen to thoes people who will tell you it doesn't make a difference at those speeds.
Bam marine has a web site where you can pull up a speed calculator that should give you a pretty good idea of speed related to HP, weight, slippage etc. Dont get to excited though with their #'s, My expierience is that they are about 5 % high

Tim

 

Boat Speed?

Guest: Some of your numbers don't make sense. If you recorded 4600 rpm with your 23" prop and then got up to 5000 with a bigger prop (28" pitch), your tach may not be reading accurately. This isn't abnormal, so you may want to get a more accurate tack or have yours tested and re-calibrated. Your calc's (based on your numbers) are correct, otherwise, so your method of calculation is right. Generally, 25% propeller slip is abnormal, and unlikely with your boat.

If the hull's bottom is not perfectly FLAT and very smooth, the resulting performance will be VERY significantly impacted - particularly at higher velocities - certainly at the speeds that you are desiring. The ideal surface is DEAD FLAT and very smooth (600 grit paper) but NOT polished (ie: no wax). If you don't have such a surface a 10mph reduction is not unreasonable at all. BUT, this loss of speed would NOT show up as propeller slip, rather it would be reflected in an RPM loss.

If you give me some more information (and some photos) about your boat design, I may be able to help by running your design through my performance analysis software. Just give me a call.

/Jimboat
AeroMarine Research

 

Whats the reason behind the not polished surface Jimboat?

 

Surface finish

The idea of "sanding" the running surface of the pads is to obtain a "smooth" (no perturbations) but "roughened" (not slick) surface. I would not sand with any more than 200-300 grit (I know my last post said 600, but it's a typo). If it gets too "slick" it will be like "waxing" and the (small) advantage of sanding would not be recognized.

When it's top speed you're looking for, polishing or waxing can make it slightly worse. The "slick" surface will maintain a "laminar" water flow for a little longer on the wetted length, which generates slightly more drag than a separated, turbulent flow. This is the same as the "stickiness" experienced when you "polish" or wax the running surfaces of your boat bottoms.

Also, most racers "smooth" the gearcase by wet-sanding with fine emery cloth. This will take out all the scratches, but won't create the "shean" that causes additional drag.

/Jimboat
AeroMarine Research

 

Jim,

I make no claims to expert knowlege of hydrodynamics but some of your statemants don't jibe with other material that I've read over the years. Most "experts" claim that the actual surface coating has little or nothing to do with drag as long as it's smooth and fair. I have heard that wax increases the surface tension and may slightly extend its effect into the boundary layer and therefore increase drag. Your medium-rough sanding sounds like the stuff that Dennis Conner put on Stars And Stripes at Perth. This was ticklish stuff applied in sheets and the "grooves" were precisely engineered in orientation and size for particular speeds. It's notable that no one uses them any more.

When the long chain polymer craze came out in the late 60's, I tried that and ran some experiments. No reduction in drag was apparent unless the material was released into the boundary layer. The ruling bodies found this out also and ruled out such injections but still allow the coating. The long chain polymers do work and are used in enclosed fluid flow systems to reduce drag.

I've always felt that a 320 grit sanded surface was good enough but have no reason to think that a 600 or greater grit surface would create higher drag.

These arguments do not include deliberate roughness or "trip" devices to induce turbulence and reduce vortex formation on high drag shapes like sailboat masts or cylindrical structures, but that is another story.

 

In very simplistic terms, lumps and scratches on a hull can be viewed as bits of foreign material stuck on the hull which cause appendage drag, in addition to the laminar drag the erudite gentlemen above have referred to and suggested sensible solutions for. At 80 mph, appendage drag near the bottom of a offshore powerboat would approach 80 lbs per square inch, or an apparent horsepower loss of around 17 HP. A rough, unfair hull can easily present several square inches of drag surface to the slipstream, resulting in large reductions in speed. For example, a 1/16th inch ridge spanning four feet transversally across the hull bottom will present three square inches of frontal surface, resulting in 240 pounds of drag. By all means, fair the hull, and also make the transition from hull bottom to transom as small a radius as is practical as this is an area where drag is introduced if seperation of the slipstream from the hull is not clean.

 

drag

Tom28571: There's always been alot of different views on the effects of surface finish on planing surface efficiency. It will clearly be different on non-planing surfaces (like Stars And Stripes). The work I've done seems to indicate that smooth, not polished surfaces to the best job of combined friction and profile drag reduction. The long-polymer experiments done usually compare to highly polished surfaces, and then they come out better. After all the work done by all the experts, there is still undecided results, so it remains a personal opinion. I have found that 300 grit surfaces work better than polished ones.

mmd: I agree with your view on drag. Your way of considering "ridges" as appendage drag is a very graphic method to show the drag from non-smooth surfaces.

/Jimboat
AeroMarine Research