How much engine?

[Brian]

Is there any formula or rule of thumb for calculating how much you can hang on the back of a boat engine wise? I've seen little boats with big engines and big boats with little engines.

I'm rebuilding a 75 hp Chrysler (cause that's what I got to work with) to go on a 17" mahogany ply cruiser. I'm sure that'll be plenty of power but it's one heavy son-of-a-gun, around 225 lbs. Add a fuel tank and a kicker motor and I start wondering how it will sit in the water and if I'll need ballast up forward.

Any ideas?

thanks,
Brian

[John Berg]

Brian, You may want to put ballast up forward anyway. The people in the know will weigh in on motor size but I can share my personel experience. I own a 20' Skagit cuddy and put a new 90hp Etec on her. The bow was to light and it bounced alot going over waves giving it a rough ride. I put 3 60 pound bags of concrete in the bow and it changed every thing. The boat cuts threw the big ones with a much smoother ride. Have fun, John

[Tim_Mattson]

Brian,
Having built boats that where just scribbled up on paper,then cut out and made up as we went along,there are many things that can be done to get a good balance.
You do want some weight to balance a huge motor but try to make sure it is at least secured somehow.
Any weight not in the lower middle area is considered "Bad Weight" while any weight below the floor and in the middle is called "Good Weight".
The farther and higher from center ,the more it acts on the hull.
All boats have an amt they will Displace and also an amt they will carry as a Load.
Remember any weight used as ballast will subtract from the Payload potential.
My first SAFE boat had a Bow Livewell I used as Ballast.....until the pounding weight Bent the bottom when ran across the salt at higher speeds than intended.
Glass won`t bend over time(ok ,trailers will bend them....) but it can loosen things up if trailered on bumpy roads that can beat up your boat worse than any water.
Batteries and Fuel tanks are good ballast to move forward.Just keep wiring as short as possible......and any lead pinned down somehow.
When in doubt,take some Trash cans boating and try to fill them up to simulate your load and waterline.Rope 4 together and bail with 5 gallon buckets.Try one up front to see if it helps or makes the bow wallow.
The shorter hulls are not as forgiving as the 20`s.....
There is a CG formula too....and after 1966-7,Wt + HP Ratings on each hull.
TM

here ya go

[John Berg]

WHat am I missing

20 x 8 = 160 I have a 90 hp Can I really put another one on? John

[Brian]

The boat has twin batteries mounted at midship, each side, just in front of the seat bulkheads. They're basically on the hull. That seemed to make sense at the time. This is the motors view forward. It has power trim, that should help.

It's 17' long and 7' beam at its widest. So according to the chart, with a 5' stern, 75 hp is toward the upper limit, but not over. So a small kicker 5 hp or so and I should be ok.

Really appreciate the advice. I'm planning on getting that boat out on the Columbia the summer. (soon as I get this motor back together)