prop size question

[megbert]

I have a 32' voyager, twin 350 crusaders that hit WOT at 4200-4300, cruises ok at 3600 but likes 3800 the best at about 18 knots. OEM says 18x18, but on the boat for the 5 years I have owned her are 18x19. However, in a box when I bought the boat are 18x21, wondering why and how would the boat perform with those on? Im ok with overall performance, would like to see it get on plan sooner, but thinking these would not help. Thoughts?

[bud37]

Heh there...a good prop shop should have a calculator to help you tune your set up but if you want to get on plane sooner the lower pitch would be the direction to go in. IMO if you were to go with the 21" you may have trouble getting it on plane but would increase your low rpm hull speed.

[buster53]

You are slightly over propped with the 18 x 19. 18 x 18 would probably be a better fit. 18 x 21 would definitely be over propping and not a good idea. Personally, I would like to see an extra 100-200 RPM’s at WOT.

BTW, 18 x 18 will get you to plane just a tad quicker.

[Midnightsun]

The correct prop is the one that will allow your boat to achieve close to, or the figures stated for WOT. My engines are rated @ 3500 RPM, I am able to achieve 3550 RPM. This is a good thing knowing the boat fully loaded is not stressing the engines due to oversized props.

[Viper]

You're right Hans. You must take the engine manufacturer's rated WOT spec for a given engine model into consideration. You can play with prop specs to improve an area of the performance curve based on your boating needs as long as you stay close to that WOT spec. Providing the engine is tuned properly, bottom is clean, boat not overloaded, etc., not reaching the WOT spec indicates you're over-propped and overworking the engine. Exceeding the WOT spec means you're under-propped and causing an engine overspeed condition which is not good either.