Picking the right motor size

[SFC Hall]

How can you determine proper motor size?

Is there a formula with the hull size / weight?

If I stand on the transom and the boat is still stable does that mean I can put a 200lbs motor on there?

This might sound stupid to some, but it you have been reading my posts, you know I am brand new to boating and I know next to nothing…. So I could use all the help I can get. Oh, and type slow so I can understand

[RodB]

This is a somewhat subjective issue (except for making a boat just plain unsafe by over-powering it or overloading the transom) on boats and the most basic approach is how much will the completed hull weight and when loaded with people and gear, what kind of performance do you want... (edited to add assuming the transom is plenty strong enough to handle a good range of motor weights). There are charts showing suggestions on this and transom strength and motor weight are difficult to consider so boat weight and payload come more into play here...But within reason most experienced folks have owned boats in the past and have figured out long ago what they felt was decent performance and speed for a boat they were using within the structural safety margins of course.

Naturally you can overpower a boat and that can be dangerous. A good example here for you to get a sense of this...would be one of the old Bass boats, the Kingfisher (fiberglass) hulls that were about 15 feet LOA (probably weighted 350-400 lbs or so) and were powered with a 50 hp Johnson. I have seen them with a 70 hp outboard and a 30 hp outboard. Some guys want to go as fast as possible we're hoping within a reasonable safety margin, others are fine with less speed but still decent performance. I think the 50 hp was perfect and ran the boat about 30-35 mph no problem with two people.

So you see, it can be quite subjective...and the manufactured boats always come with a "maximum size allowable outboard" plate on the transom. Naturally many guys cheat on this as long as they can get away with it but if a Game Warden stops them they will be ticketed for too large a motor on that boat. Many manufacturers are conservative in this venue due to liability but mostly they are taking into consideration the transom strength, motor weight, and overall weight of the hull and a reasonable load of people and gear.

RB

[Peter Eikenberry]

You didn't give any specs on the boat. It depends. For a monohull outboard boat under 20 feet, with a 20 inch transom and remote steering the formula is

(centerline length X maximum transom width X 2) - 90. Then round it up to the nearest multiple of 5. For boats 20 feet and up you can use what you want but the formula works up to about 22 0r 23 feet. After that it's better to do a manuevering test. The American Boat and Yacht Council has one, and ISO has a different test. Both work well to determine what is the best HP.

For boats that are under 20 feet, flat bottom or not remote steering there are seperate formulas.

you can see these at the Coast Guard's Boatbuilders Handbook http://www.uscgboating.org/safety/boatbuilder/index.htm

[RodB]

Interestingly enough, when I had my boat inspected for a state registration (Game warden applied a metal sticker on transom) of a "home made boat" I specifically asked about outboard motor size restrictions and was told that was pretty much up to me. I assumed the home made boats were so few that no particular rule had been written pertaining to them...only registration for the hull to be titled, same for the motor and trailer.

RB

[cjp63]

It's been my experience that the designers will usually state the range of motors for their designs.
Chris