Help chosing a prop

[Bonstrosity]

I recently picked up my first boat and am having trouble choosing a prop. It is a 1986 U.S. Marine Force 85. In the manual it says HP is rated at 5000 rpm and opperating range is between 4500 and 5500. It also says about a 13 inch prop but what pitch should I get...What do the different pitches do? I suppose change performace. What do you recomend for a prop?

 

[roscoe]

Re: Help chosing a prop

A higher pitch prop puts more pressure or resistance on the engine.<br /><br />A lower pitch prop puts less resistance on the engine.<br /><br />You need to get a prop that puts the correct amount of resistance on the engine, so that the engine operates in the recommended rpm range at WOT.<br /><br />This number will change depending on the size and weight of the boat.<br /><br />Does the boat have a prop now?<br />How does it perform?<br />What size is it?<br />What are the rpm's at wide open throttle (WOT) ?<br /><br />Without this info, its anybody's guess.<br /><br />You want that engine to run a little over 5000 rpm at WOT with a light load in the boat.<br /><br />Pitch:<br />Pitch is defined as the theoretical forward movement of a propeller during one revolution -assuming there is no "slippage" between the propeller blade and the water. For most boats, there is slippage and therefore the distance advanced is less than the design pitch. The amount of slippage varies from boat to boat.<br /><br /><br />

 

[Bonstrosity]

Re: Help chosing a prop

I don't have a tach but at WOT my boat w/ 2 gas cans and me in it will do about 35. It planes right out, it seems like it's lacking on the top end though.

 

[Gary H NC]

Re: Help chosing a prop

Nice illustration of what pitch is there Roscoe!<br />Bonstrosity, A higher pitch usually gets more top end speed as long as your engine is staying in the WOT range..You will loose a little hole shot. its give and take..<br />You should find the numbers stamped on your existing prop as for what size it is..

 

[walleyehed]

Re: Help chosing a prop

It would also be best to prop for Max RPM with a NORMAL load for best over-all performance.<br />Even though the book says 4500-5500 as a range, that was on fuel that was available in that era.<br />Due to that reason, it's best to use 5500 as the MINIMUM of the RPM range. The engine will last many years longer if you set it up for 5500-up.<br />Tell us what boat you have and maybe we can get a better Idea as to what you need.

 

[Bonstrosity]

Re: Help chosing a prop

Ok, the boat is a 1986 Bayliner Capri 1600. So it's 16 ft, with the 85 horse force. Right now something I found kind of weird is that I think there may be some parts missing b/c the prop will slide back and forth on it's shaft like a quarter inch. Is there a parts diagram for these? How about getting that locking nut off of there? There isn't a spot for a pin on there, just a locking nut. I was on a site that had composite blades that are replaceable and that could be switched w/ different lengths and pitches. Are these any good?

 

[walleyehed]

Re: Help chosing a prop

There should be a washer, such as the big one pictured in this link, that goes on the prop shaft BEFORE the prop..<br />http://www.mercurypartsexpress.com/PartsExpress/ProductSelection/Accessories/getProduct.asp?CID=168150190060227055275&lvlOneKey=74&sku=835258K+2&store=Accessories&compName=&compSKU =