prop size

[malagaoth]

i posted a while back about dropping the engine size on my 20ft semidisplacement hull from 60 to 40(thanks to all who offered advice)now i have decided to go ahead with the change as the old unit needed lots of work done to it and my question is, what size of prop? the prop calculator programme i used suggests a 11 1/2X11. this seems very small in the diameter i thought big heavy (2000lbs) boats needed large diameter props and 11 1/2inches seems very small - a 14inch will fit on the engine. any one got any ideas? i'm reluctant to jump in and maybe get the wrong thing.<br />WLL=18ft weight=2000lbs single 40hpO/B @5500rpm 2.07:1 gears

 

[Silvertip]

Re: prop size

The 2.07 gear ratio does not allow use of a large diameter prop especially with only 40 HP. I run a 40 HP, 2.42 gears, 11.5x13 SS prop on a 16.5 ft boat with three aboard and full equipment would come close to the weight of your rig. Speed with that load is 27 MPH @ 5250 RPM. If your boat is not a planing hull the ll.5x11 is probably the correct choice or very close to it.

 

[ivar]

Re: prop size

Motors in the 40HP range usually uses props ranging from 10.5" up to 11.5" in diameter, to fit props ranging from 13"-14" you'll need to look at some of the "bigfoot" models.<br /><br />edit: I read your previous post, The 60 Yamaha you got has a 12:28 gear reduction and usually was delivered with a 13x19" or 13.25x17" prop, more suitable for 16-17" planing hulls. To me it sounds like you're runing the wrong prop, and going down to a 14x11" or 14x13" would work much better. <br /><br />What motor(s) are you considering, 2stroke/4stroke etc?

 

[malagaoth]

Re: prop size

considering a 2st or possibly one of the cheaper (tohatsu or selva) 50hp lpdi 2st.

 

[ivar]

Re: prop size

How old is your Yamaha and whats wrong with it? <br /><br />If it was me, I think I'd either spent some money getting the 60hp up and running again, with a tach and a new prop, or considered something like the Yamaha FT-50 high trust. If most of your use is at lower rpms , I think a 4-stroke would work well.

 

[malagaoth]

Re: prop size

the problem is that i am technically inept! so all the work needed would have to be done at a dealers and with the price of labour and the lowerleg work plus price of a prop quotes are comming in at over 50% of a new 40hp if a respray is added to this(the boat is on a swinging mooring so paint work looks pretty tatty quite quickly) it becomes even more marginal. i do not do long journeys so top speed or fuel economy are not big issues and it would take a long time to recover the extra cost of a 4st over a 2st.