Fuel use ?bigger is better

[rik_elliott]

Just a quick one chaps

I have a caribe 13 with a 30hp 2stroke mariner from the 90s. The rib is plated up to a 40hp (doesn't state max transom weight however).
It goes ok with the thirty, but takes a bit of leaning forwards to get over the hump with three big lads on board in any real chop.

I was wondering about putting a 40hp 4stroke on the back at the end of the season. Would my fuel use go up (more horses to feed), stay the same, or go down (frugal 4 stroke?).

If the plate doesn't state a max ob weight is it ok just to stick to the hp limit?

[9D280]

Bottom line is if you want the gee gees, you have to give them something to drink!

If you spend most of the time with the throttle wide open, yes, you'll drink more. 4- strokes only really come into their own economy wise if you spend a lot of time at lower speed.

other thought is check the weights of you current vs new 4-stroke engine. Ypu may find the power - weight ratio doesnlt improve, and the extra is tright where you donlt want it! (at the back).

As a sideways thought, is your prop OK for what youre using it for?

[rik_elliott]

Prop just standard alloy prop seems to perform just fine with two on board struggles that little bit with hole shot with three though in the rough. I did find when we were hit by some evil wind on tide action near the needles it dropped off the plane quite readily with the three of us on board - it was either blast along and destroy our spines or displacement speed. Top end speed absolutely fine. I stuck some doelfins on and that helped a little but not really enough, hence my wondering if more horses was the way. Maybe smart tabs would do - cheaper for sure - it's just the thought of drilling into my lovely hull that worries me!

The current motor is very light in comparison to my old 40hp 2 stroke I had on the zap cat so yes i guess power to weight ratio would take a big hit if stepping up to 4 stroke.
Boat is mainly used as a Solent cruiser, running at about 80% throttle most of the time, so fuel efficiency is quite important to me.

[9D280]

Might be worth seeing if you can borrow a pitch down then & see how it behaves. I was with Kev on the trip to Easdale - SR4 / 40Hp, ran a 15" on the way there, was a bit lethargic with the waves &hole shot, changed for a 13" on the way back, handled like a dream. Thing was we never got the full speed potential with the 15" because of the waves.

May be worth thinking about, then just change prop for what you are doing that day?