propellor selection

[atom2000]

Hi
need some advice on the propellor selection

I tried the boat today and wto only succedded in reaching about 4200/4300 rpm,The speed was tremendous which i really do not need.
The engine is a evinrude etec 90hp ( manuf specification rpm range 4500/5500)fitted with standard manufaturer prop if not mistaken 13.75 x 17 aluminium 3 blade. The boat weights appx1100kgs which include 2 adutls and a child (40kgs) aux motor and fuel.
My intention is to have the motor run about midway of manufacture recomandation i.e about 5000/5100 Doesnt matter if i loose top end speed.
I also noticed that about 2800 the boat is already on plane.
What do you suggest I go for a 15" or a 13"
thanks

 

[steelespike]

Re: propellor selection

Can you get us weight in pounds,Type of boat/style, metal or fiberglass,
Speed in mph and max rpm.Need speed and be sure of the prop size.A rough guess a 17 should be pretty close.
I would think more than 4,300 rpm.How is the hole shot?You may want to check your tach setting.

 

[atom2000]

Re: propellor selection

weight 2425LB propellor 13.75 17 fiberglass
engine max rev 5500
regarding tach the engine etc are new used only once.
thanks
i did not record the speed by it would be in the region of 30-32 mph

 

[junior1113]

Re: propellor selection

first thing i would do is work the setup. where is the captivation plate on lower unit in relation to the bottom of boat?? this always helps to make sure it is as high as it can go without blow out. work this out before just buying another prop. average 2-4" above bottom. this means so much on performance and rpms. agree with steele 17 should do this. what year is this motor brand new? are u sure this is a 17 sounds like its a 21? give us the #'s on prop.

 

[steelespike]

Re: propellor selection

Did you confirm prop size? Your rpm seems low. You should probably go down to a 15.If you go down to a 15 You will gain ABOUT 400 rpm Putting you at about 4,700.Still way low compared to similar setups.2510lbs,16"pitch,5450,35 mph.2520lbs,15",5,500,37mph.I feel something is not right on your setup.Even though new it wouldn't be the first time the throttle isn't opening all the way or a plug wire is off or the motor is too low.or the prop a different size than you think.??

 

[atom2000]

Re: propellor selection

The cavitation plate is about 2 inches from the bottom when motor fully down.It is installed about 1 inch up from face to transom. The motor is new rigged in april by our dealer who knows his bussines. (its the 4th Johnson/evinrude I owed from them.) Yes the prop is 17inch. To be fair i only tested the engine for about 15secs since its new and only trimed the engine 1/4. Should have at least trimed it slighty higher than 1/2 way. However this was just a short test to get a idea. So went to our dealer and exhanged it to a 15 inch. Will have to check it out properly once i give the engine some running.Further more they told me to see how it goes and if required they change it for a 13inch. when considering rpm gained 4300 maybe another 200-300 with triming and 400-500 due to pitch i may get close to 5000.what do you think should i aim for 5500. The prop given is alum 3 x 13.50 x 15
Thanks for your patience

 

[atom2000]

Re: propellor selection

Hi
sorry for the delay in replying but had some teething problems which now where sorted out so now i am focusing on the engine.
I changed the prop to 13.5 x 15p 3 blade alu and fully trimmed up i got 5000 -5100 revs 2 persons on board 33/34 mph . but when encountering a wave etc there is prop cavitation if correctly stating i.e inc in revs. so backing off is required.
my question is if i go for the 13p evinrude props the revs will inc by 400 and top speed reduces which is not much of a problem. but hole shot torque how does it compare. economy wise would it be better and if the boat is loaded would i be better.

others details
about 20/22mph at 3500 rpm crusing
27mph at 4000 crusing fast
boat weight 2425LB 17 feet fiberglass day cruiser
engine max rev 5500 etec 90hp


thanks in advannce for the help

 

[hwsiii]

Re: propellor selection

Atom, don't change the prop yet, if I remember corretly Dhadley said something about the computer wont let the prop reach but so many RPM before 10 hours and then there was another increase at about 20 hours I believe. But lets give him some time to hopefully see this post and tell me if memory is correct. I do have CRS you know, but I think I am right.


H

 

[atom2000]

Re: propellor selection

thanks. I will let you informed in due course. we will be hitting bad weather force 7 wind this weekend so i have to keep moored.