16' crestliner w/1976 70hp johnson engine hight ?

[xtraham]

i have a 1978 16' amf crestliner 1976 70hp johnson w/13 1/4" x 17" prop it will crank out @ 5100 rpm's gps showing 30+/- mph a/v plate is about 1/2" below the keel would it be worth the effort to raise the engine a hole or two if so how much gain should i see? the engine will turn a 13"x19" prop but i have to trim up so much that it cavitates thanks

 

[Crownie2]

Re: 16' crestliner w/1976 70hp johnson engine hight ?

i have a 1978 16' amf crestliner 1976 70hp johnson w/13 1/4" x 17" prop it will crank out @ 5100 rpm's gps showing 30+/- mph a/v plate is about 1/2" below the keel would it be worth the effort to raise the engine a hole or two if so how much gain should i see? the engine will turn a 13"x19" prop but i have to trim up so much that it cavitates thanks

If it was me, I'd give it a try...
Sounds like the 17" is far better than the 19"

Bob

 

[xtraham]

Re: 16' crestliner w/1976 70hp johnson engine hight ?

raised the engine up 2 holes gained around 100 rpm's (5200), no increase in speed and cavitation in a hard turn, tryed bottom trim hole, 2nd trim hole was best 3rd trim it was getting ready to fly then the wheel lost bite big time this was with a 13 1/4" x 17" mich. prop tried my 13" x 19" stainless, ran about the same speed 27-28 mph but rpm's droped to around 4000 / 4100 looks like I need to come down 1 hole on the transom and try a 13x15 or maybe a 13"x13" what do ya think? just trying to get this thing `to run in 5500 rpm range would like to see 35 mph if thats possible with this set up

 

[xtraham]

Re: 16' crestliner w/1976 70hp johnson engine hight ?

update" I went to merc prop selector results were "engine to small for boat, boat to large for engine" do I realy need to go buy a bigger engine for this little boat to perform? I have seen as low as 50hp engines on the same type boat and the seem to perform fairly good. the boat is not heavy I have removed all the foam and replaced the ceiling with 1/2" plywood glassed in a couple years ago

 

[Crownie2]

Re: 16' crestliner w/1976 70hp johnson engine hight ?

I tried the calculator using your speed, pitch and RPM info and it said: No props found.

So I guess that won't help.

Yes, move the engine down one hole - you don't want blowout problems on turns.

I don't have any experience with your engine so I can't say what the recommended WOT RPM is. Maybe you know. If it is higher than 5,200 then you could try a 15".

Remember - this is never going to be a speedboat with 70Hp - but it's nice to have it set up right.

Bob

 

[xtraham]

Re: 16' crestliner w/1976 70hp johnson engine hight ?

Thanks crown, I am just trying to get the most out of it, I have heard so much about outboards dying because of lugging with todays fuel, just trying to get to cruise @ over 5000 rpm's so it will live, I would like to get wot to be about 5800 rpm's and not lose any speed.