Speed/slip problem

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.

 

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

 

Propellor trim

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

 

Performance

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.

 

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.

 

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

 

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

 

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

 

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

 

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

 

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

 

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

 

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.

 

tom lapthroplose of power

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