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Have repowered 1987 Model 18 Guardian with a 150hp Honda, Only Getting 3900 RPM & 34.5 MPH, Supposed to get 6000 RPM & 42 MPH, Weighed boat 4020 LBS, Included 28.5 gallons fuel = 175#, Engine 485#, 3 batteries 165#, trolling motor electric 60#. Prop 15.5 x 17 stainless. Does the boat have to much water in hull to make it overweight? Do not have owners manual showing the proper weight for 1987 18 Guardian. Or would boat still be able to hit 6000RPM overweight?

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There may or may not be an issue with weight. You are certainly radically overpropped. What prop do you have on there, and how did you choose it?

Tim

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How do you know the boat weighs 4020 pounds?

If it does, it is grossly overweight. I estimate a 1987 Guardian 18 (specified hull weight of 1750 pounds) with the described equipment and fuel should weigh about 2700 pounds.

Even allowing a few hundred more for equipment you may have forgotten to add, that leaves the boat half a ton overweight.

And yes, that could account for the profound loss of speed.

If you have forgotten to take into account the weight of a trailer, then it might not be so bad.

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Misquoted weight: 4200 lbs includes trailer 920 lbs, 28.5 gallons fuel 175 lbs, 3 batteries 165 lbs, Honda 150, 485 lbs, trolling motor 60 lbs, front seat 20 lbs = Net Wgt 2200 lbs.

Boston Whaler Commercial Sales Guy told me they used a 15 x 17 alum prop for the same rig setup. I purchased a Michigan Wheel 15.5 x 17 stainless to use on this set up.

Trailer and boat were weighed at certified scales at recycling center, took off trailer hitch, then weighed trailer same way without the boat.

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I"ll bet that Honda 4-stroke weighs 50# more than they are telling you. All of the engine manufacturers, to differing degrees, understate the weight of these big 3-star engines.
Then you have add weight of gear oil, engine oil and prop.

One magazine actually weighed a Merc Optimax, Yamaha V-max and Evinrude E-tec, and found all to be heavier than advertized. Mercury cheated the least (15#), with Yamaha the most(about 60#) and Evinrude in the middle(about 40#). So if they are doing it with 2-strokes, you can imagine they are also doing it with 4-strokes.

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Where is your engine mounted?

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Cut it open! I just did mine and pulled 20 gallons of water out. I have 87 18 outrage with a 2010 merc and get 44 mph with a 14x 17 prop

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Long shaft motor raised to highest level.

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Where and how did you cut the hull and how long to drain 20 gallons of water?

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OK, it sounds like you are 550 pound overweight.

When you say the motor is at the "highest level," do you mean you are using the lowest bolt holes on the motor?

What condition is the bottom in?

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OK, right there I see a problem. There is no way a motor with a 2.14:1 gear ratio (which is what the Honda BF150 uses) can spin 3900 RPM and hit 34.5 MPH with a 17" pitch prop.

Are you sure you are measuring speed in MPH and not knots? You are using a GPS aren't you?

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Right using the lowest holes and the bottom is clean with new bottom paint stored on trailer or hydrohoist.Would 550# make it not hit 6000 RPM?

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Wait a sec,

4200 - 920 - 175 - 165 - 485 - 60 - 20 = 2375 pounds net weight, not 2200.

That's 625 pounds overweight, and yes, that could account for the loss of speed.

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4020 - 920 -175-165-485-60-20= 2200
Mistakenly put 4200 should be 4020.

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I looked at your pictures and see the extensive process envolved, no easy task.

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I am using GPS will double check the knts /mph, and will get back to you exact prop make & size, I have tried two different set ups, I also wondered if the mechanic set up the rpm gauge properly as he seemed to adjust it to the sound of the motor when we were on water testing, thanks.

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4020 - 920 - 175 - 165 - 485 - 60 - 20 = 2195 pounds net weight.

OK, that is 445 pounds overweight. Not so bad.

If this boat is supposed to go 42 MPH with a 17" pitch prop then I calculate it to top out at 39 MPH with the added 445 pounds.

But this assumes it is propped correctly.

I still do not see how it could even go 34.5 MPH at only 3900 RPM with a 17" pitch prop. That is negative 17.6 percent propeller slip. Not possible.

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Stuwl wrote that his 18 ft 87 whaler is proped with 14x17 prop and gets 44 mph. I would be happy for 40 right now shows 34.5 downwind fully trimmed 1 person.

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The Outrage 18 is 500 pounds less than the Guardian 18.

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Well, I note that 34.5 knots is 39.5 MPH or about what we would expect from a 445 pound overweight Guardian 18 with 150 HP on it.

Still doesn't explain the 3900 RPM.

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Check your Tach setting? I believe it should be at 3p for the Honda. It sounds like your tach might just be reading wrong.

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Whaler says I may have water trapped around my gas tank. I am trying to lift out the floor, I have taken out all the perimeter screws, comes up to about the center console and seems to stop coming up. Do I have to take the center console screws out to? To lift the floor.

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You need to remove the console to get the tank cover off. There are 8 screws located under the console that really hold the cover in place, if you have enough play you can pull the console forward, undo the screws and lift the back. I would be very surprised to find 50 gallons of water trapped around your tank. Something else is going on with your setup, other than water around the tank. your boat may be slow with the extra weight, but your engine should still turn the prop to the rated RPMs. Check your tach and the prop, you may be mounted too high, a photo would be very helpful.

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We have a 1985 18' outrage and about two years ago we put a new 150 honda 4 stroke on and immediately had it replaced with the 115 hp engine as it was way too heavy. As soon as the dealer gave us the keys and we started it up I noticed that the water line was 2 inches lower and the boat had lost its self bailing ability. We didnt even leave the dock! Had it changed right away to the 115 and that works perfect. So I dont know about your speed issues but i do know that the 150 four stroke is grossly over weight for the 18' outrage even though the hull is rated for 150 hp max. The Gaurdian is a heavier hull but I think the 150 honda is just too heavy for an 18 footer.

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I would like to see a photo showing the stern of the boat at rest. Where is the water line in relation to the stern through hulls?

What holes in the motor top bracket are being used? I would hope they are in the third set (counting down from the top)

What is the brand and size of prop. A model number will help.

Did you weigh the trailer separately to get the weight?

I would not start opening up the boat till all the baseline info is clear.

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I agree! with the 150 on the drains in the aft were underwater!