What is the best top speed I can hope for?

Headhunter · 05 April 2004, 09:30

My first outing yesterday very pleased with set-up so far.

Humber 5.5 Destroyer with 90hp Johnson 2 stroke + OMC stainless teflon coated 19 pitch prop.

We achieved 38.5 knots (GPS) at WOT 5000 RPM flat conditions and at slack water some headwind say 6 knots and 2 persons.

From the Johnson specification data

Full throttle range is 4500-5500 rpm

Propshaft power is 90hp at 5000 rpm

Can I expect to and if so how can I improve top speed?

John

Manos · 05 April 2004, 10:31

Although I am not expert to increase the speed on our boats we have done the following:

a. Lift engine up one noch as you still don't get full revs @ 500 since your max is 5500
2. If engine over revs at that hight (i.e. over 5500) increase prop pitch (in your case from 19" to 21").
3. You need to play with props too. Same 19" pitch props do not give same top speed or performance.
4. More importantly you need money to spend

Boating is an expenssive hobby and business too

Pete7 · 05 April 2004, 10:51

I agree with Manos, as it will get very expensive to go faster so thats probably your lot although it is a good speed. You also risk turning a usable 38 knots into 40 knots in calm water but awful in the rough stuff because the prop keeps ventilating.

Pete

Manos · 05 April 2004, 10:54

Yep 100% with Pete on this one

Andy · 05 April 2004, 11:23

I used the same setup for years, and 38knots is about as good as we ever got. The boat was very usable in rough conditions and 38knots is not slow!

Have fun.

timboli · 05 April 2004, 18:59

I dont know if this helps as I am short on experience but when I tried my assault to check the pitch of the prop(new engine 30 second blast only),the speed increased by 7mph by trimming the bow up.

tim griffin · 05 April 2004, 19:08

38 knots is very good i get 45 knots on a calm day and trimmed up thats on a 7 metre with a 150 opti and a 21 prop and that is plenty enough for a training rib, usualy cruise around 25 to 30 on a good day and less if the weather is not being kind
tim
www.griffmarinesevices.co.uk

Headhunter · 14 April 2004, 11:51

My second outing on Sunday.

Humber 5.5 Destroyer with 90hp Johnson 2 stroke + OMC stainless teflon coated 19 pitch prop.

First outing we achieved 38.5 knots (GPS) at WOT 5000 RPM flat conditions and at slack water some headwind say 6 knots and 2 persons.

From the Johnson specification data

Full throttle range is 4500-5500 rpm

Propshaft power is 90hp at 5000 rpm

Second outing 5 adults 4 kids age 4-8.

Speed achieved 29.5 knots (GPS) but WOT only reached 4800 RPM
My question is:-
Is the WOT decrease to be expected with additional weight?

Comments appreciated
Thanks

John

Manos · 14 April 2004, 14:36

Roughly speaking yes.
More weight means more load on the prop therefore a high pich prop (in your case 19") will not reach WOT revs. You need to go down (say to 17") to reach WOT revs with high loads....