1984 Merc 70 HP runs well, but no top end speed

JPD63

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Sep 3, 2015
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Hello all,

New member, first question! I recently purchased a used 15' Royal American tri-hull in very good shape that is powered by a 1984 Mercury 70 hp outboard. It runs well (starting, idling, over-all reliable thus far), but does not go above 25 mph with just a driver, and does not seem to hit plane. I tried a couple easy improvements (changed plugs, fresh gas + Sea Foam), and no improvement. Brought it into my local Merc dealer/repair, and dropped~$700 on replacement of rectifier (it had literally melted), stator (used, but apparently good shape), and new CDI box, along with replacement of crispy wiring. Just took it out last night for a very brief run (it was dark and I had inadequate lighting), and I couldn't tell much of a difference. To be fair, it was a very short drive, but should've had time to plane out/hit maximum speed as I was WOT for ~ one minute. Also, I'd had to jump start my boat battery with my car battery. I should also note I have no idea what condition the carbeurator is in, and the triple prop has sizable chunks missing in each prop. Compression testing revealed pressures of 115-125 psi for all three cylinders. Also, from the scant data available about the boat itself (I'd never heard of Royal American; I now know they were built in Warsaw, Indiana for about 4 years), I believe it's weight minus motor is ~ 900 pounds.

I would greatly appreciate and insights/recommendations for improving speed/performance anyone can suggest. I'm heading up to the lake for one final week-end fling tonight and am hoping to get it up to full speed!

Best,

JPD63
 

FMan

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Aug 11, 2015
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By the sounds of it, it very well could be your propeller. It's amazing what a difference is made by the condition of the propeller. Honestly, I think just repairing or replacing that with make a huge difference. It has in my past boating experience.
 

JPD63

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Sep 3, 2015
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thanks for the help! I"ll give it a try and post results in a few days.
 

Chinewalker

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Any chance the boat is waterlogged? Not unusual for a tri-hull of that vintage to have water in the foam flotation...

Also, those 70s like to rev. If it's propped incorrectly and not allowed to peak around 5500 RPM, it simply won't get up and run.
 

JPD63

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Sep 3, 2015
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Thanks for the suggestion...How would I check for the foam being waterlogged? Also, it sounds like more support for looking at the prop...
 

sutor623

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Go to NADAguides.com, look up your boat year make and model, and it will tell you the dry weight of the hull. It will also tell you the weight of the motor. Add em up plus the gear on the rig and take er to the scale and see what she clocks in at. If it weighs 4-500lbs. more than it should, you have waterlogged foam.
 

JPD63

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That's what I thought. Thanks. If waterlogged do you have to rip up the floor to replace?
 

sutor623

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That's what I thought. Thanks. If waterlogged do you have to rip up the floor to replace?


If the foam is waterlogged, you probably have bad support stringers and transom as well as the flotation foam. We are going to assume right now that you have a different issue :)

It is also possible that you have a prop hub slipping. You can take a magic marker and mark the rubber hub and metal of the prop, take her for a spin, and then trim the motor up and see if the lines line up. If they line up, your prop hub hasnt slipped. If they do not line up, you have found your issue.

When a prop hub slips the motor will reach high rpms and the boat will not be travelling anywhere near top speed.
 

JPD63

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No Title

Threw on a new 12 pitch Merc prop last night, and will test it out tomorrow morning. Fingers are crossed.
Has anyone else ever heard of the line of Royal American boats built in the early/mid 70's?
 

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JPD63

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Thanks for the suggestion Sutor623. Hopefully it runs well tomorrow and the issues is solved already!
 

JPD63

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Sep 3, 2015
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All rightee... while the motor generally started and ran well, speed increased only marginally based on the boat's speedo and my GPS app. Driving solo with WOT, it touched 27 mph (best before was 25), and it did do a better job maintaining speed when pulling ~ 220# of kids on the tube. However, part of that may have been due to trimming up the motor a little bit. The boat is planing out, at least partially.

I've now put as much into the motor as I did in buying the whole rig, so with boating season ending soon (I can still go out on the Mississippi for another month or so), I'm not too motivated for more spending. I'll try to get a weight on the boat if I take it out again, given the possibility of waterlogged foam. I will gladly consider any further suggestions, as I'm down to: 1) the boat is too heavy, 2) carburetor, 3) timing, 4) replace the motor, or 5) sell this rig in order to move up to a better/faster boat.


Thanks in advance for your thoughts/suggestions!

JPD63
 

Chinewalker

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If it's pulling tubes and pushing you to 27mph, I'd say the motor isn't the issue. At 27, you're definitely planed out.

What is your RPM? And we do need a weight on the boat.
 

JPD63

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Sep 3, 2015
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The mechanic who replace the rectified, stator, and CDI box didn't check the RPM. Is that something most shops would be able to look at?
 
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