Too much HP??

[Mnemonic]

I have a 1964 15' Chris Craft fiberglass with a dead johnson 70 HP on it now. I got an opportunity to put a 90HP mercury straight 6cyl on it now, and I am afraid its too much for the boat.<br /><br />I cant find the Max HP plate on the boat anywhere.. Any suggestions here? Could i put it on and just not punch the motor to the max?

 

[Triton II]

Re: Too much HP??

Hey Mnemonic, you could try contacting Chris-Craft at:<br /> http://www.chriscraft.com/main.aspx?id=224 <br />They may be able to help.

 

[Mnemonic]

Re: Too much HP??

I dont know what the max is... It may be that I am allready over the Max HP with the 70HP... my last 15' boat was maxxed at 65HP..<br /><br />Is it so unsafe to go over the max by 10-20HP?

 

[jtexas]

Re: Too much HP??

I bet the Merc is well over 100 lbs heavier. <br /><br />But if you're looking for a rationalization: if 70hp is 80% of max, then max is 88 - rounds up to 90. Johnson made a 90 in 1964 so it's not out of the question.

 

[ZmOz]

Re: Too much HP??

Even if you wanted 90hp I think there are other motors that would be a better choice. Inlines drink way too much gas just for 90hp.

 

[Mnemonic]

Re: Too much HP??

Triton - Thanks, just emailed them.

 

[AMD Rules]

Re: Too much HP??

Sounds like maybe you shoud've kept the 60 Merc you originally had on it. An old 90 inline will likely be well beyond what the coast guard allows for that 15 ft boat (don't forget to consider the boat's age). You're probably the best person to judge the condition of your transom. Since you have asked the question, it sounds like you may have concerns.<br /><br />Going over whatever number Chris Craft supplies you will be irresponsible. If you ever had any issues while on the water, you could be in hot water instantly. Insurance agents will pretend they don't even know you. They put HP ratings on boats for a reason. You'd be a floating liability.<br /><br />The HP rating is not only a function of top end speed, or acceleration. Is has to do with the additional weight, and the bending forces placed on the transom. Holding the throttle back from WOT isn't the same thing.<br /><br />my 2 cents

 

[Mnemonic]

Re: Too much HP??

AMD - thanks for the info - The 60HP merc was a different boat that was sold a little while back...<br /><br />Honestly wish i kept the motor if i knew i was getting another boat.<br /><br />I will wait to see what chris craft says... I honestly dont know the transom condition well on this board because i have owned it for only a month or so...<br /><br />Thanks for the advice.

 

[AMD Rules]

Re: Too much HP??

lol... you sound like me..<br /><br />I've gone through 3 boats in 1-1/2 years.<br /><br />Boat safely

 

[Mnemonic]

Re: Too much HP??

ZmOz - I am picking up a 1980 Mercury 90HP with 2 lower units, controls, power trim & the works for $600 negotiable from a local guy retiring from boating.<br /><br />Compare that to spending $1200 before shipping on ebay for the same motor or maybe a smaller one, I'll pay the extra cost in gas.<br /><br />AMD - Yea, i tend to hop from project to project - I used to go thru 10 cars/year... now its boats and bikes.. i went thru 5 harley's last year!

 

[AMD Rules]

Re: Too much HP??

I'm sure you will find other members who will recommend that you simply turn the '9' on the cowling upside down. Good luck.

 

[Mnemonic]

Re: Too much HP??

LOL... I have been looking around like crazy for links to compare the weight of a 1977 Johnson 70HP to a 1980 Mercury 90HP... I want to know if there is a big difference.

 

[ZmOz]

Re: Too much HP??

A 90hp inline 6 should weigh around 300lbs...at least that's what my 150 weighs, and it's basically the same.

 

[AMD Rules]

Re: Too much HP??

I don't know how accurate NADA is, but dry weight for a '77 70HP Johnson is listed as 235 lbs. The '80 90HP Merc comes in at 300lbs, as ZmOz indicated. Approximately 28% increase in weight, for a 29% increase in power.

 

[Mnemonic]

Re: Too much HP??

AMD - I dont really car about a power increase - although it is nice, I want to use my boat as soon as possible and not pay alot to get it going.<br /><br />I literally just handed the slip owner $1400 for my slip today before the **** went down with this motor...<br /><br />But i do want it to be safe... I will have my mechanic check out the transom... or is this something i could co myself?

 

[dajohnson53]

Re: Too much HP??

Do a web search for the US Coast Guard site which gives a formula for figuring the maximum HP for a hull. Has to do with overall length, transom width and height, type of chines.

 

[AntsGrady]

Re: Too much HP??

Ya I'm thinkin even the 70 is maxed out for a 15' let alone 90. Whats the worse that could happen? Guess it could be to heavy for the transom. Sell the 90 and get something smaller.

 

[Mnemonic]

Re: Too much HP??

According to the USCG Backyard builders formula:<br /><br />Length - 15'<br />Width - 6 '<br />Multiply width x length = 90<br /><br />Transom Height 20" x 2 = 40<br />Add it all together and subtract 90 is the max allowed HP.<br /><br />So that means the max HP will be 40 HP on this boat? Thats a little crazy going more than double.

 

[jtexas]

Re: Too much HP??

hold on there, what's that formula? Don't make sense to add 90 then subtract it, you'll always end up with only the transom height criterion.<br /><br />I've got a 1980 15foot fiberglass boat, the max hp is 85.

 

[ZmOz]

Re: Too much HP??

That formula is pretty stupid anyway...what does the dimensions of the boat really have to do with HP? The most important factors to consider are the weight of the motor, the strength of the transom, and the speed at which that motor can propel the boat. None of those are taken into account in the coast guard formula.