2 questions, prop and hydro finns?

[chestercheese]

hello, i have 2small inflatable dinghys, the first 1 has a 3.5hp tohatsu with a 2blade prop, was wondering if its best to get a 3blade prop? and if so what performance would i achive?
second question is my other inflatable has a 7.5hp johnson, i can get it up on the plane but i have to lean forward for about 30seconds on full throttle before she's up, would hydro finns help me up quicker, is it worth puting finns on both outboards?
many thanks dan

 

[steelespike]

Re: 2 questions, prop and hydro finns?

imo inflatables don't plane as easily as a ridged hull. So your at a disadvantage right off.
I think with a 3.5 you need to be careful selecting a 3 blade.While it might handle a load better the wrong pitch may make it struggle.When you add a blade you add drag.
Careful selection of pitch is more important than the number of blades.The motors need
to be able to operate near the top of their rpm range with a typical load.
To do it right you need a tach and a speedo or gps.
An induction tach will work on both motors or perhaps an optical tach.
While a foil can help the boat plane if it doesn't clear the water on plane it can cause drag and may make the boat handle strangely.
You might consider a tiller extension so you can sit forward. The 7.5 will plane much easier and stay on plane better.With the right ridged hull the 3.5 might plane with an extension and surely will do better on the inflatable.
I've seen a jon boat on youtube that planes with a 2 hp.with the operator in the middle seat.

 

[chestercheese]

Re: 2 questions, prop and hydro finns?

hello and thanks for the info, the 7.5hp is on i ridgid hull and was hoping by putting the finns on then it would help me not having to move forward to get it on the plane? the 3.5 is on a normal inflatable and thought by putting a 3blade prop on with the same pitch then it would help move it along quicker??? let me know what you think?
many thanks dan

 

[steelespike]

Re: 2 questions, prop and hydro finns?

Adding a blade would slow you down unless you decrease pitch one incriment.Thats an estimate. It appears that is likely to be one inch.
I couldn't find a 3.5 in our prop section,only 3.3
If you can find a foil that will fit the 3.5 give it a try.It will be interesting to here your results. test each option first before trying prop and foil together.If you choose to do that.
Try a foil on the 7.5 but I think it will slow the top speed.
The foil will be a bandaid over what actually is a poor setup,too much weight in the stern.A tiller extension will allow a better balance and should improve handling.

 

[chestercheese]

Re: 2 questions, prop and hydro finns?

hello and thanks again, sorry for sounding really dum but what is the point in different numbers of blades? i understand the differences in pitch size.

 

[steelespike]

Re: 2 questions, prop and hydro finns?

A 2 blade is said to be the fastest, 3 blade is better a all around,
a 4 bade improves low to midrange. A 4 blade is good for very heavy hard to push
boats.Top end is usually slower than a 3 blade.