18 HP Johnson Seahorse Prop

[Clint1395]

i have an old 18 hp johnson seahorse on a 14 ft aluminum flat bottom boat. what is the best prop that i can buy to put on the motor for the boat i have, thanks for any replies.

 

[ONERCBOATER]

Re: 18 HP Johnson Seahorse Prop

ask this question in the prop forum and the prop experts will be of much more help.

 

[samo_ott]

Re: 18 HP Johnson Seahorse Prop

9.25x11 is the standard prop that came with most of the 60's 18's.

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

Welcome,
What year is the motor?Is the boat bare bones?

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

I believe the motor is a 57 or 58 from what ive read on here and from pics ive seen, and yes the boat is bare. got the motor at a pawn shop for 100 bucks, put an electrical in-line fuel pump on it and it runs great!! just want a tiny bit more speed out of it.

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

A stock 58 18 is good for about 27 mph on a reasonably fast boat.
When you say a little more speed do you mean lightly loaded or with passengers?
The standard prop is a 9 1/4 X 11.Weight elimination and placement are important.
Your flat bottom will help get it up on the water but may limit getting more surface out of the water and likely there are 3 keels. Raising the motor on the transom will help.You have the old style gearcase so the height will be limited.Be sure the link and sync are set right and the throttle opens all the way.
If your running really light an increase in pitch or a more efficient /speed prop may help.
The first thing you need is a tach,something like a Tinytach will work or other similar tachs.
See if you can find the prop size likely near the base of the blades. Without your rpm and speed we can't tell which way to go.

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

well its usuall just me and one other person. i have the "trim" set as high as it will go. i dont know if you would call it trim on a motor that old. lol ill look on the prop tonight and see what it says. and also i think i could get a little bit more throttle out of it, but im not sure how. i have the throttle twisted all the way, but the thing that pushes the black bushing looks like it could go about a 1/4 inch more. is there anything i can do or adjust to get more out of it? and ill get a tach this weekend if i can find one. thanks for all your help!!!

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

so i figured out that i am getting full throttle, but i couldnt find any markings on the prop.

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

Based on your description you should probably stick with an 11" prop.
A new prop might do better especially if the old prop is worn and if its original its worn.
You could watch ebay for props "speed" props occasionally pop up.
Bear in mind these special props may not do well when you add a person or gear.
Very careful adjustment of the High speed needle valve might get a few rpm.

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

yea ive got the high speed needle perfect, i close it until it starts to sputter and then back it out maybe 1/8 of a turn, then it runs great. and the prop is beat up pretty bad with chunks missing, so ill get a new one from iboats and see what it does. thanks for everyones help!!!

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

Please let us know how you make out.It woud be nice to see if the old crock will make close to its max estimate.When adjusting the high speed mix don't get it too lean.Be sure to use a 24:1 fuel mix.Use TCW3 rated oil.

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

im using 24:1 and im not sure about the tcw3 its johnson oil that i put in the fuel. however... i discovered today while fishing that someone has kinda rigged the butterfly on the carb with a piece of metal coat hangar, the butterfly wasnt opening all the way. i had it on plane and i opened the butterfly by hand and it picked up quite a bit, but when i opened it until it wouldnt go any further it would die out. even when i opened the high speed needle more it still wouldnt run right with the butterfly valve all the way open. i also noticed that the carb is starting to leak a little bit so im gonna rebuild it. my question is... is this carb designed to open the butterfly completly until it stops or what? also i noticed that the #2 cylinder is leaking from the spark plud, well thats what it looks like. does this mean that the head is possibly cracked?

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

despite my boat problems i still managed to catch this little guy.

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

Butterfly should be open fully but not beyond fully open.Check the link and sync.
There should be marks on the stator cam that the carb roller should just contact
as the stator advances.And the cam should open the carb fully at the end of its travel.
Your Johnson oil should be TCW3 rated and is likely marked as such.

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

thanks for your help!!!! i figured the carb linkage out, i think ill be getting full throttle now, i might need just a little bit more to get it to wot. and as for the oil coming out of the spark plug hole, i swapped plugs from #1 cylinder to the #2 cylinder to see if the leak follows the plug, if so ill go get new ones. however when i took them out they are both nice and brown, so i know ive been running the lean/rich mixture perfectly. i was glad to see that!!! but im about to head to the lake and see what happens. thanks again for all your help!!!!! happy fishing!!!

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

oh yea almost forgot, the line that would normally pressurize the gas tank....since i have an electric fuel pump i dont need it, i ran it out the case to vent to atmosphere, but id rather have it plugged to prevent water contamination. will it hurt the motor if i plug that line? or is it needed to vent case pressure?

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

You should plug it.
An electric pump on an outboard usually leads to trouble, can result in very serious flooding not to mention the complication of the electric source.
There is a perfectly good single line conversion. I think the instruction details are right here on the forum somewhere.

 

[Clint1395]

Re: 18 HP Johnson Seahorse Prop

yea i saw the conversion, but it seems like id have to find the right part, and do alot more work. it works fine with the electric pump so im just gonna go with this for now. i will plug the line. but all in all the motor ran absolutely perfect. now alls i need is a new prop, my old one is all ate up. but thanks for all your help!!!!!

 

[steelespike]

Re: 18 HP Johnson Seahorse Prop

OK ,good luck, let us know how the new prop works out.