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TOPIC: Max hp

Max hp 2 years 5 months ago #124253

  • dennisf
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Hello does anyone know the maximum HP engine my 1972 tomahawk 18 foot boat will take? Coast guard tag was not available back then I believe!!!any info would be great!! :laugh:

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Re:Max hp 2 years 4 months ago #125310

  • 63 Sabre
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Dennis. an 18 footer should be able to handle at least 100hp or more. My '58 14' Tomahawk had a tag on the inside transom port side just under the gunnel(gunwale). Never mind that I stuck a 1974 135hp Evinrude on the back. Goes like the dickens but sits kinda low in the stern :laugh:

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Re:Max hp 2 years 4 months ago #125367

  • Ike
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HP ratings are based on the length and transom width and whether or not it has remote steering. It's easily calculated. Just measure the centerline length, and the maximum transom width. Multiply the two. Multiply the result by 2 and then subtract 90. (L X W X 2 -90) This assumes it has a 20 inch transom height and remote steering. For more info see newboatbuilders.com/pages/hp.html

Oh Yeah, if it is not a nice multiple of 5 you can round it to the nearest multiple of 5 (example if the number is 97 then the rating is 100)

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Peter D. Eikenberry
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newboatbuilders.blogspot.com
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Re:Max hp 2 years 4 months ago #125372

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Thanks for posting that formula. I know I saw it once, but forgot how to find it. Is there a similar formula for a 15" short shaft rig?

I believe the year was 1966 when HP tags appeared if I read correctly somewhere. I know my 1965 has a tag, but no HP or load recommendations, yet my father had the 1966 version of the same boat and it DID have the HP rating plate. Correct me if I am wrong.

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Re:Max hp 2 years 4 months ago #125379

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Here's an interesting link you might want to read. It's a discussion about horsepower on boats. Keep in mind it's on the internet so it's not necessarily accurate.
continuouswave.com/ubb/Forum4/HTML/001857.html

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Re:Max hp 2 years 4 months ago #125383

I don't know if "Baby Huey" is the best reference for max HP on a boat after he over-powered his most recent project by 85 hp (270% the rating)!!! However, based on Cal's formula (more HP = FUN!!!!), I am itching to put a 1500 tower on my GT-150 once we get it into fighting shape.

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Remember - There's always room for one more project!! (not really, my garage is full!!)

1963 Lone Star Triton w/ 1979 55 hp Johnson (soon 1963 Merc 850!!!)
1968 Lone Star Mustang w/1974 70 hp Evinrude
1974 Marlin with 1971 Merc 1350
1980 Glastron GT-150 w/1977 115 hp Johnson ( soon 1977 Merc...

Re:Max hp 2 years 4 months ago #125399

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My friend was a big Barefoot waterskier, so he put a 140 Rude on his GT-150 and a 85HP hood. He ran a ski prop and turned it into a quick accelerating deep water start barefoot boat...does that translate to "run what you brung"? He had that transom beefed up pretty well.

Here are some extreme rigs that I believe have an unlimited rating.Please correct me where I am wrong. I am no expert - just ran across this stuff.
(3) 300hp on 20' long ski boat

www.google.com/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1366&bih=628&q=mad+city+ski+team&oq=Mad+city+&gs_l=img.1.9.0l10.3049.5932.0.10373.9.8.0.1.1.0.158.1111.0j8.8.0....0...1ac.1.64.img..0.9.1114.GXWGXkH7eHc#hl=en&tbm=isch&q=tripple+rig+Dyna+ski&imgrc=NTD0lQDqld5VkM%3A

another (3) 300hp outboards on a Dyna-Ski that set a world record of 60 water skier pyramid. May have to scroll down its pages.

www.dynaskiboats.com/


I think I had seen that some modern outboard boats over 20 feet long can have an "unlimited" listed on the Max HP plate. I different speed boat I looked at had that stamping. Now sure of how all or the math, etc. works.

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Dr.Go!

Re:Max hp 2 years 2 months ago #127966

Was that boat ever available with a stern drive? That will give you an idea of total transom strength. Outboards create a leverage situation slightly different from a stern drive. Plus they have a higher center of gravity.

So if installing a bigger HP outboard, it is usually a good idea to install a strong back for the top bolts that is about twice the width of the bolt pattern and is bonded and mechanically fastened to the transom. For me that means fully bedded in 5200 and screwed into the transom core to a depth of 80%.

I also suggest bolts with at least 2" washers for the bottom so that spreads the load too...

If severely overpowering, you need to take the top bolt loads to the hull stringers with braces and it does not hurt to add knees between the transom and the hull sides under the deck area.

You prolly want to post this question at ScreamandFly to see what they say :D

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