Four blades will have worse propulsion and direction control when in reverse than a three blade.

For trolling speed - I would focus on the propeller pitch, not the number of blades at that engine speed, if both props are 15 (15 inches of travel) the boat speed should stay the same.

My 2 cents - a good stainless steel three blade prop would work best – go for diameter and not too much rake.

Hi Steps

Thank you for the explanation, which i think i understand. Great way of explaining it to.

I want to clarify a couple of things as follows:

o        the only reason i referenced WOT above was to check that the new 3B prop was within the engine WOT range of 4500-5500rpm

o        my general cruise rpm is usually 3500rpm when weather permits. In terms of fuel efficiency l/h increases significantly over that rpm

based on the above i think it is reasonably well propped for the solas 3B (I’m no expert)

How do you tell if  a boat is underpowered / overpowered if its not a silly question. Some  info that may help as below: 

o        820kgs dry hull & motor

o        160kgs motor

o        boat loading

o        340kg - 1POB fuel & gear - snapper fishing

o        790kg - 1POB fuel & gear - game fishing

o        hull rating

o        max 130hp

o        no min hp stated

o        capacity 950kg

based on the above i think it is reasonably well propped for the solas 3B (I’m no expert)

If well powered .. top end yes.. generall cruise is around the 3800/ 4000.. its a balance between a few more rpms (more avaible hp at the prop) and being able to have les throttle.

the boat mechanic told me that the carb'd motor has like a three stage fuel delivery rate so to speak ie idle, lets say moderate acceleration and full accleration (not his words).

 Sort of.. sounds impressive right.. put that way its mechanic complicating things when they shouldnt...a carb be it on a 1950s 2 cyclnder jowett braford, a a 4 barrel holly on a hot rod... basically not 3 stage.. but 3 individual carbs in one.

A idle carb...when idle butterflies closed a circuit to leack fuel at a constant rate past then and the main jets.

Off idle.. thru to WoT.. butterflies open the amount of air sucked into the engine (think of it as a pump) by venturi effect sucks the fuel thru jets mixes with the air into the engine.

And acceleration.. when hit the accelerator is pushed down, the engine needs a rich mixture to speed up.. so a pump squirts more in, operated off the accelerator.

And then have the choke.. which basally chokes off air flow at idle making a rich mixture to fire up the engine on.. engines require rich mixture.. sort of like the above with the accelerator pump to accelerate.

 This is nothing 'special' for outboards or make what ever.. its just how all carbs work....not the2 mixture screws on your lawn mow / chain saw? same thing

I’m guessing that I’m in the upper range of the “moderate acceleration” when trolling at 6.5knts and rpm range of 1850 to 2000. That in tandem with the hull shape is not good if a 3B prop has more slip at low rpms.

Hang on just make another expresso

Yep the more grip u have down low the better control you have.. trolling, manoeuvring in moorings or inside a reef now one else would dare to go

A race or even max rpms set up will have around 28 or even 38% slip at troll to cruise.. WoT when the grip kicks in this drops to around 5% (see explanation compared to a race torque converter a nd shopping basket car torque converter previous post) The shopping basket (general use trolling cruise boat, at cruise you are looking at around the 10 to 14% at cruise.. trolling at the bottom end.

Side not here.. this is why so many lake boats (trout and stuff) have aux motors.. the main engine is professionally propped for max rpms and dont have control bottom end

is the boat underpowerd (guessing its not overpowered). How do you work out the 15 to 20% of surplus power that you mentioned?

s the boat underpowerd (guessing its not overpowered). How do you work out the 15 to 20% of surplus power that you mentioned?

will a 4B prop suit the boat/motor giving extra grip at lower rpms resulting in trolling speed of 6.5knts at a lower rpm than that of a 3B prop

When one buys a boat  or repowers a reputable sales person WILL ask

1/what the total gross weight will be when used 99% of the time.

2/And what the boat is going to be used or

3/And the primary expectations of the new owner troll , cruise, race etc.

4/ Then using formula works out the correct powered engine to move that weight at given rpms on the power/ rpms curve the correct powered engine plus 15 to 20%

5/ then from the gearbox ratio, the expectations of trolling, cruise wot speeds/ rpms choose the appropriate prop with the appropriate rake, cupping, pitch and diameter to meet those expectations.

6/That prop will be trailed under the full gross load for THAT customer  rpms / speed performance numbers taken and IF NEED BE the final correct prop installed.

IF any of the above is not meet , even on the 1st prop, then the engine is min or under powered for that boat....and most boats sold are in this category... wonder why boat shops become cagey AFTER the sale?

 Ok lets start doing this with your boat

 So lets take the heavy load 1st

So all up say 1650 kg with bait tank full (???)

we need to know some numbers.

Cant see them above...

Engine model type hp and gearbox ratio.

min speed at plain, at best trim up and rpms in flat as water

From there  increase approx every 1000 rpms , re trim to best speed , verge of porpoising, or just doing so in flat water.. set, do 100m let things settle in then read speed and rpms. the next till WoT.

Important rpms and speed (gps best) are accurate

The specs of the current prop brand diameter and pitch.

Then sit down and crunch some  real numbers

A larger 3 well cupped 3 blade is very similar in behaviour to a smaller 4 blade... but the larger 3 blade the outer edge speed is far greater.

Some maintain a 4 blade is hopeless at reverse..?I think this has come about because some have used 4 blades with a LOT of rake.. too much.. any prop 3 or 4 blade with alot of rake will loose a bit of reverse manoeuvrability...or if using with a P/trim ould have that effect also.

 my experience with a well chosen 4 blade ...." you can go into reefs where even jet skis dont go  in a 5.5 m cabin boat nps (subject to wind and swells.. lets be sensible here)

 So grab those numbers and lets go from there...

It is easy for people to simply say " you need this" or "this will do the job"  but so many who do dont say why.