Propellor Sizing

[mvermeer]

Hello all...new member here from Haida Gwaii, British Columbia. I have a 1981 Monk 36 with a 24/20 wheel. Power is a Lehman 135 with a Newage 2.5:1 gear. I need to run it at 2000 rpm to get 7 knots, which seems a little lacking. Can anyone tell me other power/gear/wheel size combos and their speeds? Thanks.

[High Wire]

Welcome aboard!
Typically a Monk 36 has a Borg Warner 2.57:1 trans and either a 24Dx20P 3 blade prop or a 25DX20P prop with a Perkins. You have a Lehman which not many Monks have.

The question is what RPM do you hit WOT?
Have you verified the RPM with a photo tach?

Just curious what Hull # is your Monk. Its got to be #1 or 2.

[Comodave]

Welcome aboard. Agree on verifying RPM with a photo tach and seeing if you can reach WOT RPM per specs. 2000 RPM at 7 knots doesn’t seem too bad with a single but I have twins so I can’t really compare it to my boat.

[mvermeer]

Hello High wire, thanks for the reply. I’ll get back to you on WOT and hull number. RPM has been verified, yes.

[mvermeer]

Thanks Comodave. What’s got me thinking about this is people I know with similar boats and running gear only turning up to 1600-1800 and getting 8-9 kts.

[High Wire]

Originally Posted by mvermeer

Thanks Comodave. What’s got me thinking about this is people I know with similar boats and running gear only turning up to 1600-1800 and getting 8-9 kts.

No Monk 36 or GB 36 is getting those numbers with a 135 hp engine. Cummins 210 maybe or with a 2 knot following current. I can only make about 8.5 knots WOT. My normal cruise is 6.7-6.9 knots at 1700 RPM. After that it’s about 100 RPM per 0.1 kt extra. Just a waste of fuel.

[mvermeer]

WOT is 2500RPM. To be honest, I had no idea what you meant by WOT lol! Sorry....I come from a commercial background...new to the leisure world.

[dsharp]

You have to see what the engine will turn at wide open throttle. To make sue that your wheel isn’t too large for the boat. If the engine can’t reach it’s rated rpm it isn’t good for the motor. It’s really difficult to try to compare with other people’s boat without knowing the weight of the boats to compare. Growth on the hull or the prop will kill the performance.

[Nomad Willy]

Welcome mvermeer,
So glad you’re a member now. I think you’re the only one from QCI.
I was there for 7 months or so teaching shop at Masset in about 72. On a trip to Alaska we stopped at Rupert and took the ferry across to QCC. Rented a car for several days and spent some time between Tlell and Tow Hill including the trail from Tow Hill to Hecate Strait.

You’re in good hands here regarding WOT. I prefer to do WOT at 50 to 100rpm over rated rpm. Still using about 18hp to cruise the Willard at a tad over 6 knots.
If you’re so inclined we’d all love to see pics of your boat. Five years or so there were more Monk boats active on the forum.

[rsn48]

Well you are in an area of great cruising locations. I can't help you with your prop problem but I thought I'd post a welcome here.

[mvermeer]

Thank you! Glad to be here. I have the boat up on the hard now, will do some sea trials once back in the water and get some numbers. Once I figure it out, I’ll post some pics.