Speed/slip problem

[Per S]

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

[tom28571]

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?

[Per S]

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

[woodboat]

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"

[Per S]

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

[DataTach]

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

[woodboat]

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?

[tom28571]

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?

[Per S]

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

[Per S]

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

[Richard Petersen]

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?

[woodboat]

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?

[Per S]

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

[Per S]

Here are some photos,

[Richard Petersen]

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.

[Jimboat]

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?

[Richard Petersen]

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

[Richard Petersen]

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?

[Jimboat]

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.

[DataTach]

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

[Per S]

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

[Per S]

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"

[Richard Petersen]

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.

[Richard Petersen]

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.

[Richard Petersen]

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.

[Richard Petersen]

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. :p I get it. His boat is a lot faster then mine. :p :p :p

[nevd]

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

[Richard Petersen]

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. :rolleyes:-----I am talking to myself, it is midnight in Sweden.

[woodboat]

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.

[DataTach]

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

[mmd]

Per, what is the gear ratio of your outdrive?

I ran some numbers on the following assumptions...

150 hp (112 kW) at the crank
2:1 transmission ratio
breadth at chines = 1.52m
draft, chines to keel = 0.22m
LWL, planing = 5.18m
running in salt water
max rpm = 5500

... and guess what? Projected speed is 36 knots (43.5 mph or 70 kph) with a 14D x 19P propeller. Efficiency is about 91% and cavitation coefficient is 0.93, both of which are pretty good numbers.

Using a different set of formulae (which, in my experience, have yeilded accurate pitch/diameter data but optimistic speed data), I arrived at pretty much the same numbers - 39 knots with 142 crank horsepower running through a 2:1 gearbox to a 15D x 22P prop, with a slip of 17.35%.

It just may be that you are reaching the upper limit of speed for that hull with a 112 kW motor installed. You may be able to squeeze out a few extra knots by detailing your hull bottom, but don't expect a lot.

[Richard Petersen]

Datatach is something for a person who can not read a stopwatch or a boat speedometer.

[nevd]

Hello Mark,

If I can try to clarify your comments, your Datatach is not trying to determine propellor trim (ie how far the prop shaft is from horizontal) instead you are relating prop angle of attack to slip and this will be almost independent of how far the prop shaft is from horizontal.

Please confirm my understanding.

Thanks,

nevd

[Jimboat]

I agree with mmd and Richard....

Based on your 138hp (142hp?) and 21 in prop, and 2.15:1 gear ratio per manufacturers specs, I expect that this hull is reaching it's upper limit of velocity. With best cavitation coeff's, given trim setup capability, and expected propellor transmission efficiency, I can see this hull maxing out at 45mph.

[Richard Petersen]

I dug out the specs. for a Volvo Penta AQ 145A/280 base year of 1979. Factory description of all parts of your stern drive. Good news. Those 3 holes in the trim angle plate are NOT for triming the drive. Instruction sheet has them as part of the "anti reversing pawl". The drive is always trimmed by the use of a- POWER TILT- TILT UP TO 60 degrees. 2 versions : one is trimed in NEUTRAL ONLY !! -------- Other one is " POWER TRIM" a version which can be trimed WHILE RUNNING. You are going to have to fix that motor driven tilt system FIRST. Then let the speed buildup. You should do 50 - 55 mph easy. You are 1/2 my weight-- 1/3 my frontal area--your hp is close to 75% of mine. Does not take Einstein to see who is faster. IF IF IF you could trim the drive. I can scan and E Mail the factory pages to you, to make this much simpler. You must trouble shoot the tilt drive CAREFULLY. Problem may be a broken wire only or a fuse -loose or corroded plug or connector. DO NOT check it out with the battery connected!!!!! It could start tilting by it self if you move or pull something. We are very close, time to think 2 times before any steps are taken . Parts may not be available. Call me, I repaired all types of drives for a while. I am always here to help you. DO NOT try to run the tilt motor WITHOUT A CORRECT FUSE. Stay clear of the stern drive it may open or close completly by itself!!!! It is very dangerous until it is completely repaired!!!!!!!!======You CAN NOT get it mostly working. It must be PERFECT every time. Why? If it trims IN, by itself, during maximum speed, you could, and most likely will be flipped, with the boat on top of you at maximum speed.

[Per S]

Richard,
There is no differnce in cc betweeen a b23 and a b230 but b230 is a newer motor design and stronger differnt size on berings, there more changes but so little time..

Per

[Per S]

Mr MMD,
My gear ratio is 2.15:1,
I tried wit a 15x19 prop, then top the top speed was 30knots but the rpm was aprox 7000 so to get the rpm down to 5500 I shifted prop to a 15x23 it worked hafway the rpm go down to 5500 but the the speed was only 34 knots.

Per

[oldgeardad]

Hello Per S,
First you stated you had a AQ145 motor with the 280 outdrive. This motor is rated at 145 HP at the flywheel. The 280 drive has 4 posible gear ratios Please provide me with your ratio. Additiionaly,almost all marine tachometers have various positions they can be set for depending on application . On the backside of the tach there is usually a screwdriver slotted adjustment knob that has positions such as 2,4,6,8 cylinders,posssilbly even a setting for 2 or 4 cycle or alternator pulse pickup. Obviously if this is not set correctly or is set 1/2 way between two positions your RPM reading will be incorrect. If you are turning 5500 rpm with a 21 pitch prop with the correct geared 280 outdrive I would be surprised. The 21 pitch prop is a relatively high pitch generaly speaking for your application. However, as others have informed you ,there are many potential variables involved. A boat of your weight, fully fueled with 1 or 2 passengers is doing well to run at the speed your running at. I am very interested in your situation and look forward to your reply. Thanks,oldgeardad

[Per S]

Hi oldgeardad,
My motor is not a AQ145 but close as descibed my motor is
a B230 with 405 ported cylinder head k cam, I might have some extra HP (AQ 145 have 138hp on fly wheel) but I have more torq.
The motor is from -91 except the cylinder head and the marine parts like cooling system is older.

The tacho is correct, this volvo tacho is only for 4 cylinder engines.
I dont have 15x21 I have 15x23.

Top speed is with 2 persons in the boat an less than 1/4 of gas (120 litre /4 = 30 litre).

Per

[Per S]

Richard,
VP AQ280 has not power trim, there is a sterndrive lift power by a small electric motor, on the consoll for handling this is a red light, the red light turns on when the piston from the lift isnt in "home" position. This device is not for trimmming the sterndrive.

Dont adjust trim with the lift, it will brake down.

I'm sorry Richard you are wrong, the pin is for adjusting trim.

Maybe I should get a VP 290 or a 280P for easy adjustments.

Per

[Richard Petersen]

I can not beleive Volvo and the boat manufacturer were so stupid back then to build that boat with so little trim adjustment. The information I get from you seems to change a little each time we speak. All I have is Volvo Penta --Reference number 7744---product bulletin for the ---AQ145A/280. If the drive has been greatly modified the wrong way ( removing the STANDARD and or the DELUXE POWER TRIM functions ), There is absolutly no safe way to trim the drive. You are willingly playing with a dangerous boat, and I want no part of it anymore. You have no email. You must get the documents on your own. Email me when you are ready to follow a much safer way of doing this. Rich

[Per S]

Richard,
The trim on VP 100 has only manual lifting of the sterndrive,
VP200-280 has electric lifting model called P has power trim others has manual trim like a old outbord engine.
VP has Power trim as standard.
The olnly way to adjust the trim is to manualy move the bolt to another holy, there are only 3.
I have manuals for the VP280.
I fail to see the danger of the VP 280 sterndrive.

Per

[Richard Petersen]

I can not help you any furthur. Be much more careful. The right prop should, will, put you in danger with your lack of a power trim setup. goodby and good luck. Richard.

[ljwhisenant]

tom you are losing all your power . because your boat is hydro planing on the water which means you are losing speed and rpm. to solve your proble you are going to install two hydro planing stabaliizers on you boat .you will gain back all of your power and your boat will go faster and your rpms should be where they should be