Speed/slip problem

Slip vs Velocity?

Per S - You may want to check some of the data recorded for your testing. My records show that the Volvo AQ145/280 has a normal gear ratio of 2.15:1 (0.465), and is rated at 138 hp at a recommended 5100 rpm. If you are turning 5500 to 7000 rpm, you have obviously attained some increased performance from your engine somehow? Or perhaps your tachometer is misreading (not uncommon) - so you could check that your tach is reading accurately....this is important for performance testing. Also check your speedometer for accuracy, perhaps using a GPS or Radar gun.

If indeed you are turning 5500 engine rpm and actual velocity of 34 knots (39mph) with a 21 inch pitch propeller, my calculation shows a slip of approximately 21% - so your slip does seem excessive - but I don't get 40%. (You also said your prop was 15X23in pitch...so I'm not sure which numbers you are referring to regarding the propeller used.)

Based on normal engine/drive specs and the pictures of your boat, I would not expect too much more speed from your rig. When properly setup for performance, with best propeller selected, you should be able to get slip to less than 15%, thus achieving maximum speeds in the region of 45mph (39knots), I would think. Your boat does not have a "pad" for "performance" planing, however, so I think you are nearing the maximum velocity at these speeds. Perhaps you can provide some additional information regarding the engine/drive specifications, since normal VP specs show 5100rpm/138hp?

As for performance setup, your "X-dimension" (drive placement) has the prop shaft lower in the water than normal for higher performance operation. But this is difficult to change at this point. If you are convinced that power is not a problem, then you should be able to go to a higher pitch propeller to achieve higher velocity. I think either your rpm or speedometer or drive gear ratio data currently stated may be suspect.

As for theory of setup, you can check the pattern of "wetted surface" as the boat accelerates. When properly setup, the length of wetted surface for a vee-bottom should steadily DECREASE as velocity increases. Are you seeing this? If not, then there is a setup problem (eg: x-dimension, trim angle, weight distribution, power shortage, etc).

Let me know if I can help any more?

 

You have a much newer trim adjusting bracket Dark shiny grey. It could be incorrect.And you are now running out of adjustment.

 

JIMBOAT. My Chaparral described earlier has a 14 1/4 X 21 X 3 blade ALUM @ 4800 rpm and does 50 GPS. How does he swing a bigger prop, higher revs, and go slower ? Unless his prop is worn down?

 

RPM/Slip

Richard - your numbers are closer to what I would expect with that engine. RPM seems more in line also. Your prop slip is in perfromance range also. I suspect that Per S also has some discrepencies in his data numbers.

 

Jimboat - Your observations of possible data discrepancies are on target.

Has Per S verified the Engine RPM with a digital shop tach?
Having a certified prop shop measure the actual profile of each of the propeller blades
would ensure correct pitch is used, not merely the pitch that is stamped on the prop hub by the manufacturer.
Has the MPH been verified by GPS?

The DataTach Gauge (www.datatach.com) will ensure accurate boat speed as well as prop slip, theoretical blade path, actual blade path, and propeller angle of attack are all recorded and tabulated accurately, based on actual prop pitch.

For those in the USA in February 2005, we encourage and invite you to visit the DataTach display at the Miami International Boat Show, Miami, Florida, USA, February 17-21, 2005. Convention Center Booth T48.

Per S - Have you had an opportunity to calculate the Center of Gravity? Additional fuel / water capacity, or gear, might be outside the optimal range as designed by the manufacturer. A profile photo of the boat running at WOT might be of value as well.


mark@datatach.com
Product Development Manager

 

JimBoat,
My mistake its a 15x23 not 15x21. You are correct about the gear ratio its 2.15:1. The motor is tuned. Its a B230 not a B23 as the AQ145. All bearings are new the crank is polished, the cylinderhead is from a torbo engine and its a "405" slighty ported and the cam is a K model and it has dual Solex carbs. I would like to stroke it but its expencive, you can get up to 3.0 litre or more.
About the rpm, its correct and the speed is measured with a GPS.
About the gear ratio all other gear ratios using volvo would get me more speed.
Do you think that the stern drive is too low, dont you get slip if its too high?
Is it possible to change something under the water line that could help me?
The bow goes down slighty a +28 knots.

Per

 

Richard,
What do you mean with "You have a much newer trim adjusting bracket Dark shiny grey" I dont understand.

Since you have a V8 you have a differnt gear ratio so you can go faster than me with a "smaller prop"

 

Your photos. I enlarged the top center and it very clearly shows a brand new looking GREY SHINEY trim adjusting bracket. That is a possible reason for not enough trimout holes to run right. Check it out carefully.

 

PERS. Do I read you correctly? you have done a number of HOP UP changes? That could raise HP a lot, 50, maybe with luck ? I say you are way under sized for the engine for absolute sure!!!! Put on a prop to slow the engine down to 5100 rpm maximum, with you alone. And it should fly. Borrow from a prop shop. Leave at least 2" between the edge of the blades ends and the upper plate. Keep us posted. It will do a lot better.

 

Have another boat follow you around out there . You will find a prop that bites very solid and it may push the boat to a skid or flip! You are crossing into a more dangerous speed range. Have the other boat out there with you. Very cold water up there now. Be very carefull till you have the engine at 5100 rpm alone in the boat. I want to see some warm water pictures of both of you.

 

What is the CC difference between a B23 and a B230? you may drive us to women and drink with these little changes made to the original motor. AAAAggghhh. I get it. His boat is a lot faster then mine.

 

Datatach

Mark,

I have looked at your site but can't see how the tach measures actual prop trim which is a combination of boat trim and prop shaft to boat trim.

Could you please explain for the forum.

Thanks,

nevd

 

I really would like to see a straight stick held firmly against the bottom of the outside exhaust tip. I have a bad feeling about it at wide open throttle.NAAHHH, just get the engine down to 5100rpm and everything else is peanuts. -----I am talking to myself, it is midnight in Sweden.

 

As far as drive too low in the water: Everything I've read recommends the cavitation plate flush with the bottom of the boat. From the photos your drive appears to be a little lower, causing more drag.

The bracket seems to lack enough angle/holes to me as well, agian purely from the photos.
Lasted a cupped stainles prop will hold on much better and ventilate less, If you get one go more pitch.

 

nevd,

Propeller angle of attack (actual prop trim) is tabulated, based on the difference between the theoretical blade path (slip approaches zero), and actual blade path. While the gauge does not display prop angle of attack, both the theoretical and actual blade path can be determined from real-time data obtained from the DataTach Gauge. I'll review the log later today, for data to estimate angle of attack.

Regards,
mark@datatach.com
Product Development Manager