boat wont plane

[josht52302]

I have a 1985 larson 17' open bow with a 1963 mercury 650 4 cylinder 2 stroke. I cannot get the boat to plane. The lake we go to is at 6428' elevation. I have tried different trims, loaded, unloaded. Getting pretty frustrated. Please help! The prop is a quicksilver 13 1/2-11. # on prop is 48-3039b-a3-11.

 

[SkiDad]

Re: boat wont plane

First check your compression in all 4 cylinders. If that is good drop down to a 9 pitch. Which I think is low as you can go.

 

[Texasmark]

Re: boat wont plane

Is this a new event? If not, 6000' can take a lot of whoopie out of your engine. I went over the 8500' mountain at Alamagordo, NM. on a trip a couple years back and my Dodge 1500 P/U just fell on it's face....really it had no power to speak of.

650 is engine rated, not prop rated so in today's terms it's only 85%ish of that hp. So, a 55hp on a 17, even at half HP should plane out a LIGHT semi-V and run 20ish at least...planing speed being around 15 to 18. But that's at sea level. If you lost 50% like I know I did with my truck at 8500 you are under powered. If you have her loaded down with stuff and passengers and have a deep V all the way to the stern then that just makes matters worse. Do you have PTT? If not, where is the tilt bar....which hole? What are you running for a prop?

There is a formula for altitude vs HP and some of the old salts on here know what it is. I don't as I'm at 800' and don't think about it.

Mark

 

[josht52302]

Re: boat wont plane

The compression is good and even within 2-3 lbs, all around 80 psi. I just got the boat and motor. I have tried all 6 holes in the tilt. 2 from the top puts the anti cav. level with the bottom of the boat.

 

[jestor68]

Re: boat wont plane

You lose about 4% for every 1000 ft in elevation above sea level.

Anything below 100 psi is considered low compression on that Mercury motor than read 135-145 when fresh. So questionable compression plus about a 25% altitude power lose equals double trouble.

Does the boat have a tachometer? If so, what rpm can the motor attain?

 

[Texasmark]

Re: boat wont plane

In my service manual for the 3-4 cylinder Merc 75-125 hp engine it says: ....Expect trouble for readings below 120 PSI. Agree that having the readings constant cylinder to cylinder is a good measure, but 80#?????? No way Jose!......as the saying goes.

Mark

 

[craze1cars]

Re: boat wont plane

HP loss calculator for elevation: Wallace Racing - Braking HP Loss at Altitude Calculator
Shows your 65 horsepower engine loses 12.5 hp at that altitude, so first recognize that at your altitude you have a 52.5 hp engine, if it is in perfect running condition. There is NOTHING that can compensate for this elevation loss except installing a turbo or supercharger...it's just gone.

And your compression test, if also performed at that altitude, needs a correction factor of about .83. So your 80 readings divided by .83 = approx 96 per cylinder true compression if you did the test at 6300 ft.

While better than 80, 96 is still not a good compression number, and if the test was accurate it indicates a severely worn engine. HOWEVER compression readings vary greatly by what kind of compression tester you have. For example the 12" long flexible hose found on many cheapo compression testers can reveal drastically low numbers on small cylinder engines (like your 4 cylinder 65) due to the volume of the long hose falsely reducing the compression ratio of each cylinder. You can only obtain accurate compression reading with a VERY short hose (or no hose at all) on a compression tester, when testing a small cylinder engine such as yours.

Anywho...according to NADA, the 1985 sterndrive versions of the 17' Larsons ran a 120 hp motor as standard. So I'm curious what does the Coast Guard placard on your outboard setup say for maximum HP rating? If it says 120 or similar, the answer to your original question is real simple...a boat rated for 120 will REALLY struggle to plane if it only has a 52.5 hp motor installed...especially one that is likely making even less than 52.5 hp...

Knowing your RPM at full throttle would be extremely valuable info.

There are a multitude of factors conspiring against you here...but I am fairly convinced your engine is simply waaaayyyy too small for your intended use and you'll need to find a larger engine if you want your boat to plane.

 

[steelespike]

Re: boat wont plane

Also keep in mind that the power loss is based on a motor that has been re jetted for that elevation.
If it has sea level jetting power is further reduced.If the carbs have a high speed needle it needs to be adjusted at altitude.
Have you determined it is running on all cylinders?
While the 650 may prove to be inadequate at that altitude it seems to me that it should at least plane lightly loaded.

 

[500dollar744ti]

Re: boat wont plane

I think you need more power. Even if you can get your boat to plane, it's always going feel underpowered. What is the max rated hp for your boat?

 

[josht52302]

Re: boat wont plane

I've got a line on a evenrude 130 HP, gonna try boat at lower elevation on friday, weel see if anything changes.

 

[craze1cars]

Re: boat wont plane

I've got a line on a evenrude 130 HP.

Holy smokes! A 65 to a 130 would be one heck of a jump...I almost fear maybe too much for a 1985 17 footer? There is a very obvious placard on this boat which will show you maximum rated horsepower for this boat. What does it say? Boat is closing in on 30 years old...make DARN sure the boat and transom can handle such a jump in power and added engine weight before doing it! Read the placard, and also check your transom for internal wood rot. I would hate to see a rotted transom sink to the bottom of the lake with your big 130...followed shortly thereafter by the rest of your boat...leaving you and yours swimming...

 

[Texasmark]

Re: boat wont plane

Holy smokes! A 65 to a 130 would be one heck of a jump...I almost fear maybe too much for a 1985 17 footer? There is a very obvious placard on this boat which will show you maximum rated horsepower for this boat. What does it say? Boat is closing in on 30 years old...make DARN sure the boat and transom can handle such a jump in power and added engine weight before doing it! Read the placard, and also check your transom for internal wood rot. I would hate to see a rotted transom sink to the bottom of the lake with your big 130...followed shortly thereafter by the rest of your boat...leaving you and yours swimming...

My tin lizzie is 17' net wt 800# BIA rating plackard 130 hp with 700# BMG max. I run a 90 on it at sea level and get right at 50 mph. Can't imagine what that thing would do with it's full rated power but it must be safe....BIA said so!

Mark

 

[josht52302]

Re: boat wont plane

The 130 is also from the 60s so it probably only has 120hp, 115 at 6500'. the hull is sound, transom is sturdy.

 

[jestor68]

Re: boat wont plane

I've got a line on a evenrude 130 HP.

Steer clear of any of those early Evinrudes or Johnsons with the electric shift lower units. When they tear up(and they will) it's very hard to find parts.

Better off sticking with a Mercury motor. Generally want a motor not older than the boat.

 

[Texasmark]

Re: boat wont plane

Other thing with older engines is crossflow vs loop charged. Steer clear of crossflow as the earlier ones just loved the gas stations. Easy to tell the difference. Pop a plug and look at the piston crown. CF is domed and Loop is flat.

Mark

 

[josht52302]

Re: boat wont plane

Ok, quick update. Schrader valve iv my compression guage was bad. All 4 cylinders are right around 120psi. Gonna try to lean this pig out. I thing its running too rich for our high altitude. No high speed fuel trim? Guess I'll need some smaller jets. Might as well rebuild the carbs while im in there.

 

[Texasmark]

Re: boat wont plane

One more thing just for viewing. I just stumbled across a chart in my service manual on orifice size vs altitude.

Knowing your current jet size the reduction would be like:

Current .034", 6000' would reduce to .032

Current .048 .046