Mildly Wild flats/bay boat 15'

[Danger Oar]

Well let me say I'm new here but not new to the boating world. I wanna pick whoever's mind that will let me in on some in site. I own a 15' key west cc and just recently repowered from a 60hp merc to a 125 merc. with 22p laser prop. I'm running about 50% more hp than recommended for the vessel. That obviously does not bother me. So when adding more power i installed a 6" setback manual jack plate and smart tabs with 60lbs actuators. Never the less, I bought the motor used on good ole craigs list so in a rush i just slapped the outboard on the transom so i can get it in the water. Eye balled it and said thats good enough for testing. Motor runs like a champ. 48mph @ 5200 rpm which is right on the money for expected performance from what I've been told via email conversations via PropGod. So now i have verified the motor is good i am working on proper set up of the 6" setback jackplate. Really not interested in getting nosecone with low water pickup. Just wanna set it up with what i got. Looking for a good starting point for motor height. As i think i understand it for ever 12" of set back is 1" lift of the motor from its normal position. So I'm thinking 1½" to 2" above the keel to start and work from there. Am i right with this

[sharpeye Mike]

Add a water pressure gauge and raise it as high as you want till you loose water pressure. No nose cone here and no low water pickup, I just blocked off a few hole on top.

[Danger Oar]

Having the motor at highest possible height without effecting water pressure, how will this effect the handling at cruising speed in tight creeks?

[sharpeye Mike]

Cruising speed will be fine, plus less drag, more top end speed and more clearance before you break a skeg or prop on a stone or what ever is hiding below the water line.
If you don't like it you can always bring it back down some.
Keep in mind in that the pic. I am using a surfacing prop (cleaver) and on my fishing boat I am using a good old OMC SST and didn't raise it as high.
Keep an eye on your tac., you might have to go up a pitch or two.

[Danger Oar]

Picked me up a water pressure gauge. Gonna install it sometime this weekend. Thanks for the input

[phillnjack]

you need to get motor up a lot higher than the cav plate 2 inches above the bottom.

put the jack plate on and put the motor up as high as possible, fit your water gauge and go play with it.
let it rev, 5200 is killing it, with the jack plate and right up expect 5800 or more. but it might have a 5200 rev limiter ?
and atleast 55 should be seen with that shape hull without tons of gear on board.

keep us informed how it goes, I reckon it will be real cool .

[Danger Oar]

5250 is the max recommended rpm on my outboard. I think there is a rev limiter set at 5500 rpm from what i can gather from research. Its a 2000 merc 125hp classic.

[phillnjack]

what year was the 60hp merc and what speed did that give you ?

[Danger Oar]

The 60 merc was a 2001. Got 34mph wot. Total weight 1800lbs . dry weight 950 plus fuel, ppl, grear and motor. So when i punch in those numbers to this web site http://www.go-fast.com/boat_speed_predictions.htm , i can get a hull factor. Once i got that set i can change the HP and it predicts my top speed. Just a lil under 50mph. Im pretty close to that. 48mph with 8%slip. But think i can get a lil better by transom tuning. Once i got the engine at the best height i might can go up in prop size. But even then going to a 25 pitch prop will get me no more than 55mph if the motor will turn it. I will have more answers later this week after i get the water pressure gauge mounted and start increasing engine height.

[phillnjack]

something is very wrong with your boat set up.
My old 15 ft boat was as heavy like yours with a older 60hp merc and 10mph faster.
you are running the boat far too flat and must have the bow down far too low when wot.
first thing to do is take those tabs off the boat or atleast un-couple them and tie them up in the air, you dont need those for sure with one of
those boats.
then get engine up as high as it can go, all the way up on the jack plate too.
then take out any weight possible from up the front and put it in middle or in last half of boat.


I looked these key west 15cc up on youtube and too many people have these way too much bow down, its crazy.
and to only get 34mph from a 60hp merc is realy bad.

here is my old 1991 15ft centre consol boat with its 1991 60hp merc 15 pitch alloy, this boat was 40mph all day long with 2 people on board and loads of stuff.
If just me in the boat 44mph was easy and got there very quickly.
with the vented 15 pitch it could pull up a 200 pound skier. I did not even have a jack plate on that boat.
this ran a 90 litre fuel tank under the seat in front of the consol and battery under drivers seat. 2 live wells in the splashwell.

people that had these with a johnson or evinrude 70 would leave me for dead as they could hit 50 with ease.

.

[phillnjack]

and according to that program you use the boat should be doing 59mph not 49.
its 41mph with a 60hp. and that is giving 1800 pounds and a 125hp ..

That program is nothing at all like the real world, I just type my boat into that and I should be doing 58mph...
never will mine do that without putting another engine next to it.

[W2F a V-King]

Yes, that speed calculator is pretty generous. Would be great if it was accurate!!

[Danger Oar]

At 34 mph with the 60, if i trimmed it up any more it would start porpoising real bad. Thats how the trim tabs got added. Even when i type in the numbers from the key west data sheet (34mph 50hp 1700lbs) i come up with a 198 constant and still only get 52mph. On that program you type in your knows to get your constant first. Then you can change the other numbers. As far as getting the weight more towards the middle or back, the only thing i can do is remove the trolling motor. The fuel tank sits below the deck between the center console and the bow. I also have 2 batteries in the console and one in the Stern. I'm baffled by this lack of speed too and will continue to accept advise and continue with trial and error till i get it the best i can

[Danger Oar]

Update: after adjusting engine height I lost speed. Lowered the engine back down and gained a lil speed lost rpm. I know it didnt make since to me at the time. This lead me to believe that there was a mechanical failure. There was. Found the throttle lever/ timing lever was stuck together not allowing the butterfly in the carbs to open all the way. In fact all my results are about half throttle. Just fixxed this issue and anxiously waiting for a free day i can get it back in the water. Thanks for all that have chimed in on this. I will keep yall posted

[phillnjack]

Now you might get it up and start realy moving, then once happy start again jacking it up.

hope it goes good for you, keep us posted