TOPIC: experienced opinions needed

I'm in the process of restoring a '59 larson 15' runabout. I have a chance to get either an older 85 hp v-4 evinrude or a 115 ToP Merc. Is either of them to heavy or too much HP for that boat. It came with a seized 35hp. I was wanting a little more get up and go but I also don't want to be unsafe. Thanks for any advice.

If you check out the Larson area of Fiberglassics library, it will tell you the maximum size horsepower recommended for the year and how many feet long model of Larson All Americans. Also many other interesting facts. Most interesting is how much they cost back then.

Too much horsepower is a relative term. Too much for what? speed, safety, fun?
I had a 115hp Evinrude on a 15' Tomahawk, The stern sat dangerously close to the water when idle and you couldn't walk to the back but it topped off just short of 60mph GPS. Watching the expression of other boat owners when you pass them up in an old classic is priceless. Those ratings are recommended for safety and weight. An old OMC 115 V4 weighs a lot, a newer one is considerably less weight. The old 35hp was a good match at the time, a newer 60hp would almost be ideal for performance and safety.

I've got a '73 tri hull that's 15' with a 6' beam, with the original V-4, 85HP Evinrude. It should weigh about 275lbs by the books. The old boat runs near 30mph with 2 of us in it on a calm lake. I'm 260lbs and can easily sit on the rear decks and still have 10+ inches above the waterline. I run it in the heavy chop on the salt creeks on our coast with 4+ people in it and have never felt like it was in danger of swamping the transom.

Thanks for the info. Used motors are hard to come by (in decent shape). I'm going to try and get the Merc 115 and if it's too much, I'll save it for my '57 Lonestar which is the next project--and probably be my keeper for awhile.

Test pic

You should be aware of a few things before you hang a 115HP engine on a 15 foot boat.
Just so you know where I am coming from I spent 25 years working in the USCG Boating Safety Program advising boat manufacturers and builder on the rules and regs, and boat owners as well.

HP ratings are based on more just weight. The industry adopted a standard for HP back in the 50's which was later adopted by ABYC and the USCG and is now used pretty much worldwide. Part of it is the type of boat and does it have remote steering. The formula for a monohull boat with remote steering (A steering wheel) and a 20 inch transom height (standard lower unit length) is the centerline length (in ft) times the maximum transom width (ft) times 2 minus 90. If it comes out say 56 you can round it to the next multiple of five (60). You may have notice, weight has no factor here. It's strictly the boat dimensions. There are different formulas for boats with 15 inch transom height, no remote steering (tiller steered) or flat bottom hard chine boats, (typically jon boats)

Where weight does come in is in determining the safe load for the boat and the amount of flotation the boat should have. To make a boat float level when full of water requires enough flotation near the stern to keep that engine upright and stable and the powerhead out of the water. IF you put a seriously heavier engine on the boat it eats into your safe load and the boat will probably not float at all when full of water. So swamp the boat, which is now likely to because by putting a large engine on it you have moved the center of gravity considerable back toward the stern. That's why the transom sits lower.

Also, not seen in the formula, is the maneuvering issue. This formula was originally derived by running boats through a carefully designed test course that tested their maneuverability. If the boat couldn't get through the course safely with an engine then they had to use a lower HP engine. That test course is still used by industry to test boats and a modified version (the collision avoidance test) is used in Europe to test HP ratings.

So it goes way beyond weight. And by the way engines have gotten considerable heavier.

Now, understand, the HP rating is manufacturer requirement. They have to label the boat with a safe HP. So the FEDERAL law does not apply to you, but as usual there is a catch. Many states have passed laws making it a state offence to exceed the horsepower rating for your boat. Oops!.

What may save you from getting a ticket is many of the boats made in the 50's and 60's had no label stating HP and safe load because it was not required until 1972. But builders who were members of the old Boating Industry Assoc, and/or the Outboard Boating Club voluntarily put labels on their boats. Mine has a BIA label. Barely readable but it's still there. A water cop sees that and they will use it. Oh by the way it is against both Federal and state laws to remove or alter the label.

Thanks for the info. I was checking weights on engines just last night and decided to try and go with a 60-70 hp merc or 'rude. Safety was the main reason for me asking. I'll put the larger motor on my '57 Lonestar Holiday when the time comes for that resto to begin.

Very interesting information. My understanding is that a modern Outboard boat of 20 feet in length or more is plated for unlimited HP. This must be so because I see 20 foot ski show boats with 3 - 300hp outboards on back. Wonder how all that came about?

Poking a stick in a hornets nest here but..... I guess that just about puts all 460 Ford, Big block Chevy's jet boats etc illegal.
Government regulation gone amuck.
IMHO A REQUIRED safe boating course along with a boating license, driver alcohol regulations and plain old common sense would make gov't regs on hp ratings obsolete.
Like a lot of other things there is no cure for stupidity and unfortunatly there are boaters in this catagory.

No. Because the US govt doesn't regulate inboard boat horsepower. And Hp regulations only apply to monohull O/B boats under 20 feet in length. So yes you can hang whatever you want on your 20 foot or longer boat. However. boat manufacturers usually make HP recommendations for o/b boats up to about 26 feet. But they are recommendations. They are based on testing of the boat to see how it performs with different sized engines.

So if you want 3 100HP engines on your 20 foot ski boat, and the weight doesn't sink the boat, well go for it. I hope your insurance covers it.

For many years I had a cartoon on my desk of a person sailing off to go around the world, and the boat, if you could call it that, was another person with several life jackets on. The Caption read "Never Underestimate the power of human stupidity" A quote from one of Robert Heinlein's books. That's why the Darwin awards were invented.

As for inboard boats, naval architects and yacht designers usually do horsepower calculations based on the design parameters of the boat. Many use a series of formulas developed over many years of tank testing of models and testing of full size boats, and they are usually designed to use the optimal horsepower, not the maximum the boat could handle. In others words its a compromise of weight , power, efficiency and the designers desired speed for the boat. Obviously boats like Fountain are going to have lots of HP because the manufacturer wants them to go fast (over 100mph for the fastest ones) A bayliner on the other hand is going to have much smaller engines because the boats market is aimed primarily at first time boat buyers. The Hot Boat (Performance Boat) market is a whole different world where HP is king and economy be dam.... Speed is the desired end.

Great information. Thanks for sharing it with us all. This is why I love this site. Filled with experts that are happy to share.

"IMHO A REQUIRED safe boating course along with a boating license, driver alcohol regulations and plain old common sense would make gov't regs on hp ratings obsolete."

EXACTLY what I have said for years and years. Dealers will sell the most powerful boat in the world to the stupidest person in the world...and frequently do. Generally, the buyers have no clue about boat operation, rules of the road and don't really give a damn...period. They dunk it and dump it. Then there are the jerks who turn 6 and 7 year-olds loose on a jet ski that goes 50-70 MPH.

EXACTLY what I have said for years and years. Dealers will sell the most powerful boat in the world to the stupidest person in the world...and frequently do. Generally, the buyers have no clue about boat operation, rules of the road and don't really give a damn...period. They dunk it and dump it. Then there are the jerks who turn 6 and 7 year-olds loose on a jet ski that goes 50-70 MPH.

Well, keep hammering your state legislators to change the laws. Every state has a Boating Law Administrator (usually the head of whatever organization in your state enforces boating laws) This person reports directly to the governor. Many states have passed laws requiring mandatory education, and age limits on operating specific types of boats, particularly Personal Watercraft. Your state also may have a state Boating Advisory Committee. Find out who they are and contact them. They have no legislative power, but they do advise the BLA and the governor on boating matters.

In the meantime. speaking of engines, I need to find a new one for my boat!

How new of an engine and what is the length of your boat?

Its' a 1959 Larson 15' runabout. I'd prefer something close to period but right now, anything will do that will fit a 15'' transom and 50 hp min, 70 hp max. When I finish this one, I have a '57 Lonestar Holiday that came with a fat fifty -long gone- and hopefully will be repowered with a 150 give or take a few. Do you have something you're looking to part with?

I have a 1960 Lonestar Flamingo fiberglass that I run a V4 Johnson 85 hp. and weight is not a problem but...I DON'T run her wide open cuz the handling goes overboard!!!