Speed/slip problem

I have a 19" boat, the motor is a VP AQ 145 tuned and a VP 280. The prop is a 15x21 in good condition. The problem is that the top speed is only 34 knots at 5500 rpm, it should be much more. This means that the slip is 40%.

What could be wrong?

Per S

 

Per, No one else has ventured an answer to your problem and I am not able to do so either. Slip of 40 % on a 19 foot boat at 34 knots seems very excessive though. My thought is that the boat is just not compatible with higher speeds. If the gear reduction is, proper, there seems to be no other reason. Since the engine turns out to 5500 RPM, lack of power is not a problem and the prop seems adequate.

What is the type of hull and what is the weight?

 

Thank for your reply,
the weight is 800kg and it has v-shape, I will take some photos.

Do you know of boat that dont go faster than lets say 35 knot?

You have to excuse my spelling, I'm from Sweden.

Per

 

Without seeming smart there are of course boats that have issues going fast, just look at a tugboat for example. Is there any more info you can provide? like:
Gear ratio? how about a speed to RPM curve? I ask because it matters if say it is hitting a a wall. There was a recent poster using a surface drive that after say 34 Knots it would just push the nose over more with more throttle but wouldn't really go faster. So if you post much more data like speed at 1000, 2000, 3000, 4000, 5000 and 5500 we can see "how it feels"

 

Thanks for your reply,
My boat is not a tug... with a V shaped bottom.
We have winter a the moment so its not possible to make the test you ask for, but its a great idea. interesting what about the nose issue my boat does the same the nose goes down when going from 28knot to 34knot. Where did you see the poster?

I'm going to take some photos so you can see the shape of the boat.

Per

 

Per
Your propeller slip question is very interesting. When gathering your performance data,
you might consider a DataTach gauge (www.datatach.com). The DataTach gauge measures real time vessel speed, engine rpm, and apparent propeller slip. Recorded 16 times per second, the data are automatically saved to a multimedia card (same used in digital cameras). You can then plot your own graphs in Microsoft Excel. Sample test data from an 18' V-bottom are on the website. Includes graphs of MPH, RPM, Prop Slip, and Angle of Attack.

We encourage you to visit our display at the Miami International Boat Show, Miami, Florida, USA, February 17-21, 2005. Convention Center Booth T48

mark@datatach.com

 

It may not being completly relevent but was here http://www.boatdesign.net/forums/showthread.php?t=5157&highlight=surface+piercing
Is your prop ventilating? Is your prop cupped? Is it stainless or aluminum? Is your I/O trimmed all the way up and the nose stil pushes over?

Of course I realize your boat isn't a tug. It was meant to point out that not all hulls are capable of high speed. There is simply too much drag. So you may have a hull shape that is difficult to go fast regardless of power applied. You got any photos of the outdrive and placement on transom?

 

Ah so, maybe you and Woodboat have hit on it. If teh bow is being driven down as the boat goes from 28 to 34 knots, then it is adding skin friction, instead of the normal reduction. Most designers will say that a boat like yours will have the lowest total drag at high speed if the trim angle of the aft bottom is at 5 to 6 degrees above horizontal. The flatter the deadrise, the less trim angle is required and vice versa.

Can't say what the specific cure might be. If that is the problem, the cure is to get the bow up. Weight distribution, trim tabs, put in a bit of aft rocker?

 

Mark,
it would be nice to go to Miami but its a bit to far for me, our device sounds interesting but I know rpm, speed and gear ratio so then I know the slip.
Per

 

Thanks for your interesting reply's,
I will check the link. The propeller is not cuped, it's made in aluminum. One thing I been tinking about is the position of the sterndrive but I have checked Volvo's manuals and it's Ok unless my boat needs a special fitting.
The VP 280 has not power trim so its less fun to adjust the trim, I have tried all fixed positions and max trim is the best.
I'm thinking of making some device that gives me more trim, maybe that will help.
I hope that the maximum speed is not 34knot.
Photos will be taken tomorrow, we have a storm at the moment.


Per

 

800 kilos=1,763 lbs--- 19 feet long --gas Volvo 280=275 hp minimum -- prop is slipping on the drive shaft? Or prop is not what is stamped on the hub. Or it is, but it is a outboard prop on a inboard== lots of slip. How many blades?--------- 5500rpm does it really sound like the engine is turning that fast?-----Where does the VP 145 fit on the boat? what is it?

 

I'm thinking prop too. Aluminum is thicker generally and without cupping easily ventilates. As it does, much like the problem with the surface piercing, it will drive the nose over. I think the prop is just wrong. What was the original prop supplied from the manufacturer?

 

Richard,
VP 280 is the sterndrive not the hp, the engine is a 4 cylinder 2,3 litre volvo aprox 150hp totaly rebuided and tuned.
the prop is not sliping on the shaft. the prop is a 3 blade volvo / Michigan orginal 15x23. The rpm is ok for this engine, I run it at 7000 rpm with a 15x19 prop.

Per

 

Here are some photos,

 

Per S. I have a 18 1/2' 24 degree Chaparral SSI ( 3,500 lbs running ) 5.0 L 220 hp V8, and in perfect trim should do the high fifties. Has the boat always run this speed or is this speed after your engine was rebuilt? The rpm and prop changes look like you are not even close on the prop sizes now. When did all the over reving start? I see where you are on the last trim hole. Running a prop in a high slippage speed WILL DEFINATELY eat the edges of the prop off and make the slippage worse and worse very quickly. Take the prop to a place that has a NEW, prop of the same size for your engine.--- It should be the prop, what else can cause such a slippage? Keep us informed. Good luck-----------The only thing else is your exhaust pipe, very deep and It looks like it is angled down. Could be picture is not true enough.