20hp prop or engine problem?

[mjbf15]

I just picked up a '71 20hp Seahorse and put it on a 15' fiberglass boat. The motor seems to run great, but I am not getting the performance I expected. At full throttle the motor seems slightly under load, yet the boat doesn't even get on plane. The total load myself, gear and boat is probably around 500#s. The current prop is an old 9x10. I can't tell if the prop is trying to move too much water and subsequently the boat is barely moving, or if the prop is moving too little water and the likewise. <br />Can anyone help? Do you think it is an engine or prop problem?

 

[OBJ]

Re: 20hp prop or engine problem?

Welcome to iboats mjbf!<br /><br />Beforw we start tearing the engine apart, let's look at engine setup on the boat.<br /><br />How far below the keel of the boat is the prop? How high is the transom from keel to where the engine sets?<br /><br />If the lower unit is really far below the keel, you might be running a long shaft engine on a transom built for a short shaft. There is 5" difference between the two. Makes a huge difference in the performance of the boat.

 

[tashasdaddy]

Re: 20hp prop or engine problem?

i use to run a 1950 15hp on a 1970 14' fiberglass boat. would get up and haul a--. you have a set up problem. imho.

 

[mjbf15]

Re: 20hp prop or engine problem?

First the model# is 20R 69D and the serial# is J3108522. The distance from the keel to the bottom of the skeg is about 13". From the keel to the center of the prop is about 8". The boat is 14' long (not 15) and has a 17" transom.<br />I appreciate any help!

 

[OBJ]

Re: 20hp prop or engine problem?

Morning mjbf.<br /><br />There is a plate right over the prop. It's called the anti-cavitation plate (other things by some), How far below keel to you recon the plate is?

 

[surlyjoe]

Re: 20hp prop or engine problem?

what kind of baot is it? got a pic? 500# seems awful light for y0u, the boat and the motor

 

[WillyBWright]

Re: 20hp prop or engine problem?

If that's an old 60s fibergass runabout, a 20 ain't gonna plane it. One of those weighs 500# all by itself. It wouldn't have any problem planing a 15' jon boat. It might not be the motor, but the boat. But pull the sparkplugs and see how they're burning. If one looks significantly different than the other, you might have a motor problem.<br /><br />That 17" transom could be a problem. Are you measuring at the spot where the motor hangs? Sometimes transoms are made with a cutout for the motor to sit in. That motor is made for a 15" transom. The antiventilation plate should ride an inch or two below the waterline (boat on plane) as opposed to newer thru-prop exhaust motors which ride that plate at the water line.

 

[gatorred]

Re: 20hp prop or engine problem?

if the prop slip's you will have slow speed with it out of water put it gear and turn motor over by prop if it slip's you can feal it,get the prop rebuilt or swap for one that's right it's no going to be fast but it should get you off lake i know trolling boat are slow.

 

[tashasdaddy]

Re: 20hp prop or engine problem?

if you don't have a digital camera use a disposable, when deveolped have them make a CD. then you can post. Need pictures of total boat, back area with the motor on it.

 

[rickdb1boat]

Re: 20hp prop or engine problem?

I'm with Willy on this one. I don't think there is enough power for the boat. I had an older 60's 16' fiberglass boat and it would not come close to getting on plane with a 20HP. I had a 65 HP on there and could have used more...

 

[mjbf15]

Re: 20hp prop or engine problem?

I've posted pictures on PictureTrail:<br /> http://www.PictureTrail.com/gid9292282 <br />I'd say the cavitation plate is about 2-3" from the keel.<br />Also, with the current setup the motor is in a slight trim down position.<br />The boat was built in '71, not sure of make; theScrumbler is probably right about weighing more than 500#s. It just seems light to pick up.<br />As for needing more power, I couldn't agree more. Such is the case with most aspects of life, not just boats............

 

[rickdb1boat]

Re: 20hp prop or engine problem?

The motor looks like it is mounted a bit low. Hard to tell from the pic. That does look like a light boat. Nothing like the one I had.. Might try putting a 1" piece of wood between the motor mount and the hull and raise it up some. Then see how she does.. A drop to a 9" pitch may help, too. Looks like a neat little boat though..

 

[tashasdaddy]

Re: 20hp prop or engine problem?

that thing should fly!!! are you having this problem with just you in the boat, or with someone in the front. put a piece of 1X2 wood between top of transom and the motor mount bracket, also move tirm pin out 1 notch. put a long hose on you gas tank and move it forward. you siad in you 1st post yourself and gear. if all the weight is in the rear it will never plane. i have had boats like this. presently i have a 14' aluminum. very similar to yours and it will not plane with just me unless i put the battry, gas tank in the front, and then it's still scary. email me will send pictures. tashasdaddy@comcast.net.

 

[DHPMARINE]

Re: 20hp prop or engine problem?

Might also try moving that tilt pin up one or two notches (one notch at a time).The setup you have could be 'plowing' too much water with the bow.<br /><br />DHP

 

[OBJ]

Re: 20hp prop or engine problem?

I think everyone will agree that the setup could be better. Little higher for sure. mjbf....if you could get the anti-cav plate even with the keel, that would be great. If there are bolt holes in the transom brackets, use them to fasten the engine to the transom.<br /><br />Just looking at the engine from the pics, what do you know about it....maint. wise? It's wise to check some things on a "new" engine. Compression, spark, gear lube, just to name a few. Let us know how you make out.

 

[jfm1]

Re: 20hp prop or engine problem?

I have the same motor on my boat 14ft fiberglass appox 455lb now add 18gal fuel tank 2 deep cycle batt. trolling motor 2 seats myself 322lbs my fishing partner 300lbs fishing gear. my boat moves around good it takes along time to plain out, but it does. should note mines a tri-hull, 1969 chrysles sport valiante, remount your motor as OBJ said make sure its running right

 

[Dave Abrahamson]

Re: 20hp prop or engine problem?

Nice picture gallery. T-38?

 

[mjbf15]

Re: 20hp prop or engine problem?

Yep, the mighty Talon....I'm at Tyndall now and on to the Eagle....recall what I said earlier about more power...<br />I've placed a 1x2 to space the motor from the transom; I'll give it a go tomorrow. <br />I'll let you know how it worked out Fri evening....before beers... <br />OBJ, I took the guy's word for it that he gave it a one-over recently and everything checks good. I trust him as far as I can throw him....but he's a big dude.

 

[OBJ]

Re: 20hp prop or engine problem?

Hey mjbf....I was at Tyndall.....way back when they still had the F-106 Delta Dart on the books as a first line interceptor. I did weapons control on them...after the AC-119 gunships. Luck to ya'.

 

[itstippy]

Re: 20hp prop or engine problem?

In the pics it looks like one of the flukes on the prop is bent terrible bad. Like it was bent backing the boat into a driveway.<br />Maybe it's just the pic. But if it's indeed bent the way it looks it'll just thrash water.<br />As tashdaddy says, "that thing should fly". You can make it fly faster by working on the setup and weight distribution, but if if doesn't even plane the way it is then there is something serious and obvious wrong.