small boat prop question, 15hp Johnson

[Mike Johnson]

15hp Johnson, previously used on a 14' MirroCraft DeepFisherman, 9-1/2x10"prop (standard issue for that motor from factory). If not loaded too heavy, it would just barely plane the 14 mirrocraft. Put a 25hp evinrude onthe mirrocraft, moved the 15hp Jonny to a new aquisition:12' aluminum deep and wide (65" beam)vee bottom of unknown mfg. This is a heavy boat for a 12 footer, has a carpted flat aluminum floor,but still is not as large nor heavy as the old MirroCraft. Very poor preformance on the 12' boat. I had expected it to really move! Dealer suggested a change to a 10x8" prop. This was an improvement, but still poor preformance. Next tried a 10x7" prop. Specs for the motor are:max hp (15)@6250 rpm., WOT range 5500-7000 rpm. Several test runs with the 10x7" prop yielded 5650-5700 rpm WOT with two aboard, 6300rpm WOT solo. Still not quite planed out. (yes, motor is properly trimmed) Only one stock prop choice left, 10x5". This seems extreme for a 12' aluminum vee. Am I missing something? The dealer, too is stymied. Help and suggestions?

 

[OBJ]

Re: small boat prop question, 15hp Johnson

UP...check the position of the anti-cavitation plate to the keel of the boat. Should be even or better yet, 1" above. Will make a difference.

 

[mjbrueck]

Re: small boat prop question, 15hp Johnson

The only small motor that I've played around with different props on was a 1957 Johnson 18hp. What I noticed was that on the smaller pitch props, there was a lot more slip. I don't recall the numbers exactly. My top speed with the 9-1/4x11 prop was around 23 mph, with the 9x9 prop it was 11 mph (both at 4500 rpms - max for the engine). If the slip on the 9x9 was the same, it should have run at 18 mph. That was also partly due to the boat planing out around 13 mph, so at 11 mph the hull is very inefficient leading to more prop slip. Also tried a 9x10-1/2 and 9x10. Both had much more slip than the 11 pitch.<br /><br />You're making your maximum engine power when solo with the 10x7 (6300 rpms), so if you're not planed at this speed, lower pitch isn't going to help. <br /><br />Assuming, your boat's balanced (you don't have too much weight in the rear to get over the bow wave):<br />I'd first try what OBJ said, raising the motor as high as possible without cavitating. <br />Next I'd try lightening the boat (eliminating the deck or building a lighter one). With 15 hp you shouldn't have a problem planing up to around 1000 # (boat, motor, people and gear).

 

[umblecumbuz]

Re: small boat prop question, 15hp Johnson

Now we've got ANTI-cavitation plates!<br /><br />And I fort this were an edificated for -um! <br /><br />If'n ah fit won of em fings, will it stop the water boilin around the edges of ma prop?

 

[Mike Johnson]

Re: small boat prop question, 15hp Johnson

prop position seems to be proper.

 

[Mike Johnson]

Re: small boat prop question, 15hp Johnson

prop position seems to be proper.

 

[Mike Johnson]

Re: small boat prop question, 15hp Johnson

where are all the prop experts? I'm still working on this....

 

[Moody Blue]

Re: small boat prop question, 15hp Johnson

Doesn't sound like a prop issue to me. I think you have shown that. Can you confirm that the anti-ventilation (for you still fishing )plate is even with or above the keel ? How old is that Johnson ? Done a compression test on it ? When you say the motor is properly trimmed what do you mean ? Is it closest to the transom, furthest away or somewhere in-between ?<br /><br />That motor should easily plane the boat out with two on-board unless it is made of steel plate . Here is a long shot. Have you always owned this motor ? It is possible some previous owner has changed out the hood and what you really have is only a 9.9 ? What is the s/n on the motor ?

 

[mjbrueck]

Re: small boat prop question, 15hp Johnson

In your first post, you said you've got it propped right (6300 rpms @ WOT). What you want is to go faster, so you need to look at ways to lighten the load (or balance the boat better) to allow you to use a higher pitched prop to attain more speed. <br /><br />Also for reference, my dad has a 14' Sea Nymph (probably very similar to your MirroCraft) that he had a 1976 15hp Johnson on, and it would easily plane with two people, trolling motor, battery and fishing gear. Your motor may not be making full power.

 

[Mike Johnson]

Re: small boat prop question, 15hp Johnson

baught the motor from a friend, have original receiipt, and it is relatively low hours for an 88, last season, it would plane the 14' Mirrocraft, which is heavier than this 12'. The anti-cavitation plate (I also beleive that to be correct, it is designed to prevent cavitation) is at level of top of keel, I've tried all trim positions, one hole out from the bottom seems to be the best.I have not done a comp test, but find it hard to beleive that the compression/power output would have deteoriated in 6 mos of storage (and properly winterized, fogged, etc)

 

[cobra 3.0]

Re: small boat prop question, 15hp Johnson

My 1997 15 hp flies on a 14 ft boat even with three big guys in it. Something is definitely wrong. Are you getting full throttle with it? Check and see if the carb is opening up all the way.<br /><br />What is your boat r

 

[cobra 3.0]

Re: small boat prop question, 15hp Johnson

This is your boat?<br /><br /><br /> http://www.mirrocraft.com/boats/anglerschoice/utilityv.php

 

[Moody Blue]

Re: small boat prop question, 15hp Johnson

The anti-cavitation plate (I also beleive that to be correct, it is designed to prevent cavitation)

Incorrect. The plate is to prevent air from being drawn down from the water surface into the prop stream and causing ventilation. Hence it is an anti-ventilation plate. Cavitation is caused by extreme low pressure against the prop surface causing the water to in effect boil. There are better technical explanations if you do a search on these forums.

 

[Moody Blue]

Re: small boat prop question, 15hp Johnson

UPtraveller, after much research and investigation I have found the problem. You need to be running a MERC instead of that Johnson and all your problems will be solved Just kidding of course.<br /><br />Are you dragging the anchor ? OK enough of that What is the attitude of the boat when it is running at WOT ? Is the bow out of the water at all ? What about that S/N for verification. This is certainly a puzzle.