maximum speed of reconditioned 7.5hp volvo penta MD5B?

[Mistral53]

I have just reconditioned my volvo penta MD5B engine. I am getting 4.5knts where I expected at least 6knts. Is this top speed about right? The boat is a sunray 21. I would appreciate any welcome information thanks.

 

[smeaks]

i posted along these lines recently and found that most people get 4.5-6.0 knots in a boat of this size (displacement hull) some very good feedback re boat length and hull speed. My 22ft max's at about 5.

 

[andy01842]

I get 5 knots from my 4hp outboard. Boat is 17.5 foot seahawk, think you should be a bit faster. Is the speed over ground i.e. gps or through water i.e. log?

 

[VicS]

The maximum hull speed in knots is approx 1.5 times the square root of the waterline length. so for the Sunray that is about 6.3 knots.

If the engine reaches maximum revs at a lower speed you need to consider the prop dimensions

 

[starboard]

I have an Albin Cirrus, a classic Swedish cruiser racer of 7.8mtr LOA. This being Swedish is no light weight and my MD5B gives me 5.5kts in no wind or waves. The speed drops off if I am pushing into a wind or lumpy sea, but then again she is 27 years old. One problem with the MD5b is if you over throttle it all it does is make more noise for no speed gain and throws out loads of carbon and unburnt fuel out from the exhaust!!

Paul.

 

[Spyro]

Hull speed is approx 1.34 x square root of waterline length.
which for a 21 ft boat will be much lower than 6 knots.

 

[VicS]

[ QUOTE ]
1.34 x square root

[/ QUOTE ] Different source obviously and i wont stand by mine! The WLW is 17.5ft so that works out at 5.6kn. which is still significantly more than 4.5.

 

[Bajansailor]

As noted above, the propeller you have might not be ideal - what is the maximum rated output revs of the engine, and what revs are you actually getting?

Also, what is the condition of your bottom? If there is a bit of fouling, you can easily lose half a knot, perhaps more, especially with bilge keels (lots more wetted surface area).

 

[VicS]

[ QUOTE ]
new user

[/ QUOTE ] I missed that. Welcome to the forums. I hope that you get some useful advice.

 

[Mistral53]

Thanks everyone for your reply to my engine speed queery,many interesting ideas. My Sunray21 is a bilge keel which has been newly anti-foul painted. Unsure about prop, size and pitch, Iwill have this checked.

 

[Mistral53]

By my calculations that would be:-
1.34 x 4.58 = 6.14. please could you show how you get it much lower than this

 

[VicS]

[ QUOTE ]
1.34 x 4.58 = 6.14

[/ QUOTE ] You have used the LOA of 21 ft, you should have used then LWL (The waterline length) which according to my info is 17.5 ft for a Sunray. Then as I said a few posts further back that then works out at approx 5.6 knots using this formula. Still more than the 4.5 you are experiencing though.

 

[philwebb]

Hi Dave, I have the same engine as you with a sail drive mounted
in an 8 metre Kelt sailboat. I reconditioned it last year. After a scrub and antifoul this year I was getting 5.8/5.9kts on my log.
Have you got a saildrive or a shaft, what rpm are you doing?
at max throttle I usually do about 2600rpm. Have you had your
injector checked? If the engine is running Ok you may need a
different prop.