TOPIC: MAX HP FORMULA?

friends,

this may be a stupid question BUT is there a "formula" for figuring the suitable horsepower OB motor for old hulls that don't HAVE an OBC rating on the data-plate?

the reason that i asked is that this AM i found a DEEP/82"WIDE/HEAVILY-built, "gutted" 18.5ft, trihull/bowrider OB hull on a tandem trailer "for cheap" (100.oo for all) that was made in Miami, OK.
(the "unidentified maker" boat looks like a cross between a deck-boat & a "fish-n-ski" & according to the property owner the boat/trailer was "ON the property", when he bought it 20+ years ago.)

the data-plate has only (what i assume is) the serial number of the hull & a "seemingly meaningless" string of 6 letters (RGMFZD). - the "space for HP rating" is BLANK, i.e., UN-stamped.

i'm considering buying the sound hull/trailer, putting a suitable OB on it & "re-modeling" it to suit me for a "fishing machine" for the south TX bays.

yours, satx

I would stop at 225hp.......but then again I run a 125hp on a 13' , Its going to take 100hp to get it on plane,so If I was doing it I would start looking in that range and up, 115hp is an easy find and would do the tric.

satx
Seems as though Ike had put that formula in a post here somewhere.
I can't remember where though.
Neil

Ponyboy; neil,

THANKS.
i hope someone has the forumla, as i'm NOT exited about buying a OB & figuring out later, that it's like army drawers = too BIG or too little!
(CHUCKLE)

yours, satx

Cost of boat x 1.5

Good one Todd! I think the formula is covered in this doc;
files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf
Ike (or someone) posted one a while back, may be the same?

Most guidlines are devised around the average novice boat user. Sort of an idiot-proof system. Since it is heavy and has a deep V at the stern, a decent amount of power will be needed to push it along without straining. I'd shoot for 125 at the low end, 225 the top. Just remember, you don't have to use all of the power available. But having an underpowered boat is a huge disappointment - and it will burn more fuel.

MarkS,

according to that formula, i should use 150HP.

THANK YOU.

yours, satx

Bruce Gerard,

THANKS!

yours, satx

Bruce Gerard wrote:

Just remember, you don't have to use all of the power available. But having an underpowered boat is a huge disappointment - and it will burn more fuel.

Yup. I've NEVER wished I had a smaller motor. There are, however, PLENTY of times I had a few more horses back there.

THe formulas are VERY conservative, and don't take a lot in to account about individual boat designs. They will provide you a good starting point, and a general guideline, but not a 'dead on' figure. Of course, you don't want to really overdo it, as weight will also become an issue (have you checked the weights of some of the modern motors?? WOW!), but remember there is nothing that forces you to push the throttle all the way forward.

There is a graph that I printed out from somewhere on this site that was from the OIA about 1965. I can't find the link right now, but at 18.5 L and 82" W, the chart says max 155hp.

Andgott; thetudor,

THANKS GUYS! - fwiw, i "have a line on" a 150HP long-foot, 1999 Johnson Intruder, which is said to be COMPLETELY rebuilt (with less than TWO hours since rebuild) by a factory-trained (retired) OMC mechanic here in SA, so i MAY have found my OB.
(and it has fuel tanks, wiring harnesses, controls,everything.)

i'm supposed to go "have a look at it" on Monday, next when the mechanic gets back from a fishing trip.

THANKS again!

yours, satx

Now the motor sounds better than the boat!.....lol

Better load it in the truck and take it home so you can get a really good look.

Ponyboy; jepstr67,

well for SURE (IF i buy it) it will cost me THIRTY-EIGHT times as much cash, as the boat/motor did!
(talked to the "parts guy" at a local boat dealer today & he said that he had offered "Bob" 500.oo for just the trailer about 6 months ago.)

FUNNY how having your "ear chewed off" by an angry spouse "lowers your expectations" on $$$$$.
(CHUCKLE.)

NOTE: "the parts guy" says that the boat was built about 197? by OKLAHOMA PLASTICS, LTD, of Miami, OK & that they were a "maker of knock-offs".

THANKS GUYS!

yours, satx

Sorry I didn't get a look at this earlier. I have been so busy working on my boat while the sun is shining that I haven't been on-line much the last few weeks. But it's raining now so here I am.

The USCG formula for that type of boat is centerline length X maximum transom width x 2 - 90 ((L X X 2) -90 = HP

round up to the nearest multiple of five. This is assuming it has remote steering and at least a 20 inch transom height (19 Plus inches) Look here for the USCG formulas for all types of boats newboatbuilders.com/pages/hp.html It also assumes that this is a monohull, that is, if you drew a line around the waterline of the boat, it would be one continuous line. If it has more than one footprint (such as a catamaran or pontoon boat) it is a multihull, and that formula doesn't apply. You said it was a trihull and most trihulls of that period were monohulls.

This also applies to boats uder 20 feet but is commonly used for boats upto about 26 feet.

Ike,

the OK Plastics, Ltd. boat is a trihull up forward which transitions to a modified V astern.= i'm thinking that 150-175 HP (155 is what the formula tells me.)is about right for my needs as a TX/LA bay-fishing machine.
(i'm NOT looking for a "speed demon" OR a "deep water rig, but rather a wide/deep/stable hull that will NOT soon repeat my experience with a juvenile TIGER SHARK "boarding my boat W/O the captains consent".
(he ATE a tacklebox before we could disembark him!)

THANKS for your input.

yours, satx

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