Prop change question

[bigrockets]

I have a 17 ft center console boat with a 50 evinrude 1990. I recently purchased the boat. The previous owner used it for some fishing and mostly pulling tubes with 3 to 4 people. I intend to use it for fishing and some cruising with 2 or 3 people. I test drove it for the first time this past week. It gets on plane slowly and seems to over rev at WOT. The engine spec is WOT between 4500 and 5500. It will easily hit 6000 and the prop seems to cavitate. The current prop is 11.75 diameter and 17 pitch. At WOT the boat does 22-24 mph using GPS. It also seems to run bow high while on plane. I looked at the engine trim tube and it used to be one hole closer to the transom as evidenced by the paint wear at that hole. So it has been trimmed out one hole from where it was. It sounds to me after reading many posts here that I could go from the 17 pitch to a 19 pitch to lower the WOT rpm. The michigan wheel prop selector recommended a 12.25 diameter prop with 15 pitch which sounds equilvilent to the current prop.

I'm thinking I should first try the engine trimmed back closer to the transom and try it again before investing in another prop. The cavitation plate is level with the lowest point of the keel which ends up being the bottom of the transom. Any advice or guidance would be appreciated.

 

[steelespike]

Re: Prop change question

You would have some very serious slip to only produce 24 at 6,000.
And maybe you do;but it wouldn't hurt to comfirm the tach is accurate.
Typical prop is around a 15".Though an occasional 17 does turn up.
A 19 would be unheard of for that size boat.I have a 330 lb. boat with an 88 50hp
E, it runs a 19. Don't worry about diameter as pitch goes up diameter goes down.Diameter is seldom considered in prop selection.I would confirm the tach before any prop selection.

 

[bigrockets]

Re: Prop change question

Someone I work with suggested that maybe the prop hub is worn since the prop is old and that may be why it seems to rev. He suggested I buy a composite prop in the original pitch (17) and get a 15 pitch also since two composite props are fairly reasonable in cost. I borrowed a hand tach from a marine mechanic friend and my tach is about 30 rpm higher than the handheld up to 3000 rpm which is all I can get out of the engine out of the water with the muff and garden hose. I went to the Michigan Wheel web site and put in my boat and motor info and their site recommends a 12.25 inch prop at 15 inches of pitch. I think my plan is to get a similar new prop, set the engine back to the original trim setting and try it again. In reading the evinrude and prop forum I found a few instances of a slipping hub causing problems. Thanks for your response and help.