Help with prop selection

[AnkorCraft79]

Wanting to get a new prop for my 16' tri-hull with a 120 Mercruiser. Have a 14.5 x 18 now and with 2 people top end is 3800 rpm and around 28mph. I think I'm over proped and worry about putting to heavy a load on the old girl. Any ideas where to go from here? We always have 6 to 7 people with a top end of 3500rpm and 26mph at WOT.

 

[steelespike]

Re: Help with prop selection

what year is your boat?Not knowing your gear ratio;
by the seat of my pants your boat should be doing much better.typical prop would be around a 21" pitch and produce speeds around 40.with rpm about 4200-4400.

With your loads the pitch would be less probably around a 17 with the caution to watch the rpm with a light load.You would have to go down to a 16" just touch the bottom of your rpm with 2 aboard.Something like a 15" just to touch the bottom end
with a load.Thats very low.
You may need to do a careful tuneup with a compression check and be sure the carb is opening all the way.Is the bottom clean and smooth? It seems that 7 people would be beyond the boats rated capacity.?

 

[AnkorCraft79]

Re: Help with prop selection

It is a 1979 AnkorCraft rated for 7 adults. Does have a smooth bottom and motor is fine except for lower compression in one cylinder (85) from a bad valve. Will deal with that this winter but runs smooth and strong. The prop need comes from this weekend at the lake. Beached the boat and son didn't push off but backed it out in the mud/rocks. Didn't trash it but made it a good second prop. I think it is prealpha 1 type outdrive.

 

[AnkorCraft79]

Re: Help with prop selection

One thing I left out was this is a 4 blade prop 14.5 X 18 and I think it came with a 3 blade type because someone cut the torque compensator blade/fin so the blade didn't hit.

 

[steelespike]

Re: Help with prop selection

It probably has the 4 blade because they help hole shot and low to mid range performance. Boat is 1700 dry.Motor is rated 4,000-4,600.
gear ratio is 1.7:1 So would probably require somewhat less prop than a modern 3.0
with a 2.00:1.imo it should still approach 40 maybe with a 19" with the rpm within its rating.So your prop isn't out of line but your rpm is really low.The 4 blade may hold your rpm and speed down a little.With the load you usually carry I think I would stick with the 4 bld. Maybe a 16 should add about 400 rpm.With a valve job
maybe you'll end up close to that 4,600 figure and approach the high 30s.

 

[AnkorCraft79]

Re: Help with prop selection

Thanks for the info, valve job is planned for this winter. Think I will have the damaged one repaired ($65) and look for another 4 blade prop around the 16 to 17 pitch range.