Prop selection advise... 40hp colvic seaworker

[fatsdoodah]

Can anyone assist selecting suitable prop for mk1 22ft colvic seaworker?

It has no inboard but we have just got a 40hp yamariner 2 stroke outboard that we are fitting. The prop is in very bad condition so needs replacement.

I'm sure stock prop is probably not well suited to our heavy full displacement boat. Trouble I'm having in doing calcs myself is lack of required information for our boat.... displacement, LWL, draft etc. Sailboatdata.com is no use here!

Perhaps I could work back from stock seaworker prop spec and engine rpm then adjust for rpm of our outboard?

Many thanks for any assistance in advance!

 

[ChromeDome]

Good propeller repair shops can fix even severely damaged props



Normally way cheaper than new and you have an option to change the pitch if desired.

 

[fatsdoodah]

Good propeller repair shops can fix even severely damaged props
View attachment 170883
View attachment 170884

Normally way cheaper than new and you have an option to change the pitch if desired.

It's ali outboard prop.... £70 new with powdercoat. If repairing and re pitching is cheaper that's good to know. Still stuck with working out correct pitch / diameter though!

 

[Alex_Blackwood]

Have you tried here.
Bei Facebook anmelden

 

[fatsdoodah]

Thanks yes I just joined and asked same question there

 

[fatsdoodah]

All else being equal (design speed and shaft power) should a well mached prop decrease in pitch as you increase either engine rpm or displacement/drag?

It seems more displacement / drag will tend to slow down the engine so a shallower pitch might make sense. Similarly if prop is designed for specific boat with say lower rpm a shallower prop would be required for a higher rpm motor.

 

[Refueler]

Just remember :

Diameter for thrust
Pitch for speed.

 

[ChromeDome]

The dimension of the damaged should be stamped into the hub somewhere, so if happy with the performance (before the damage) you should just have repaired/replaced with the same.

If you know the achieved rpm you can select low pitch for more rpm or higher pitch for lower - the crucial thing is that the engine operates at full throttle within the range stated for it.

 

[Tranona]

Your basic problem is that the outboard is inappropriate for your boat. It is designed for planing boats and therefore is setup with a high shaft speed and small propeller necessary for getting a boat on the plane. Typically your boat would have a 35-40hp inboard revving to between 3-3500rpm and a 2:1 reduction to give a shaft speed of approx 1500rpm and an 18" or so diameter propeller. The outboard revs to 5000rpm and probably has a 2:1 reduction giving a shaft speed of 2500rpm and would need a propeller approx 12" diameter and maybe 5 or 6" pitch whereas a normal prop for a planing craft would be 11.5*12" because it will be moving the boat at 3 or 4 times the speed that yours is capable of.

However you only need around 20hp to achieve hull speed of just under 6 knots so you may well find that a standard outboard prop will work in benign conditions but may well struggle in heavier weather because the prop will cavitate if you apply more power.

 

[Refueler]

As soon as an outboard is mounted on a displacement hull - its usually out of its designed intention ... its why companys such as Mercury produced Saildrive Outboards ... They had heavier modified gearbox reduction ratio to have the 'thrust prop' rather than speed.

I know this is not what OP wants to read - but if it was my Colvic ... I would be looking for a reasonable 2nd hand inboard around the 25 - 35hp mark ... selling the outboard to fund it.

If not - then just plump for the same prop as the outboard had ... at 40hp - even with unsuitable prop - it will work ok.

 

[Boathook]

Yamaha did do or do a selection of dual thrust props for displacement speed boats. You do need a fairly fine pitch for your boat and you will be very limited in the range of props available.

 

[ChromeDome]

40 horses never will come to work unless in an emergency. If you pitch it low enough to reach 6 knots at max recommended rpm the noise will become an annoying companion.

Get a high thrust prop if you can, or else get a repair and push along nicely at 10-20% throttle.

 

[Snowgoose-1]

It might be worth ringing round dealers with the idea of part exchange for an outboard that may suit your Colvic .

If in good condition, the 2 stroke high reving outboard you have is still very much in demand for planing boats. Mainly sport fisherman who want to speedily go from fishing spots etc to try their luck somewhere else.

.

 

[fatsdoodah]

Sorry just catching up! Thanks for useful input folks.