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Mr. Sandman · 02-06-2003, 09:31 AM

ooh ooh... Having done some work on propeller hydrodynamics...(mainly for ships but the same principals applies) and having done a little work with some the merc marine race guys developing an integrated propulsion system/hull a few years back... let me add my 2 cents.

Small boat prop selection is sort of a trial and error game IMO. Most boat manufactures don't do the necessary model (or full scale) testing do obtain the info needed for ideal prop selection. So what they do in most cases is try off the shelf props that are in the ballpark and see which one seems to be good enough without blowing the engine at wide open. There is actually a lot to it but since cost is the driving issue, pick one off the shelf and ship it! is the motto.
You might say...how much would I gain if we had the perfect prop. You might do 10% better if you are lucky. To some that is a lot, to some...not. So off the shelf picks are the rule for pleasure boaters. Ships and competitive racing is another story.

Anyway, you generally should pick the best prop (near peak eff.) at the design speed of the boat/engine, not at troll. If you decide you want a slower toll speed and you put a prop on that gives you that (reduce the pitch), you will over-rev the engine at high speeds. For most boaters, they think the "perfect" prop is one that lets the engine run at red line at wide-open throttle and they live with the performance at all other ranges. The hard thing about propellers is you only have one pitch (or pitch distribution)and that is only ideal at one advance ratio (inflow speed in essence) BUT you have a *range* of operational conditions, no single pitch and be ideal at all rpms and load. (That is why some ships have controllable pitch props..To match the pitch to the condition.

ANYWAY, If you want a slower troll I would leave the prop alone if it were performing OK at other speeds. (The most important speed is the cruise condition)

Can you ...Lower the RPM of the engine at troll?...(It sounds like you have a 4 cycle they are much better at low RPM then 2 cycles and you might be able to adjust the low speed throttle a bit to give you a slower troll)
Also, you might think about creating drag some how...(toss over a sea anchor) Not very elegant but it would slow you down.

If you troll a lot, you might want to consider a cheap trolling motor or (kicker), they can give you a great troll, the prop is so mismatched for the boat that you can creep along.

If you have ever been on some of the newer 50 or 60 footers with big high torque diesels, they tend to troll way too fast because they need to have a big prop to absorb the power...have you ever been on some of these boats when they put them in gear and they lurch forward? A couple of the newer boats I have been on have a cool option...it is called a trolling throttle, it lets the transmission slip at low speeds to essentially let you get any trolling speed you want without touching the main fuel throttle.

Sorry for all the babble, I would try and adjust the low speed throttle and not #^&#^&#^&#^& with the prop first.

02-06-2003, 09:31 AM	#10
Mr. Sandman Registered User iTrader: (0) Join Date: Sep 2001 Posts: 7,649	ooh ooh... Having done some work on propeller hydrodynamics...(mainly for ships but the same principals applies) and having done a little work with some the merc marine race guys developing an integrated propulsion system/hull a few years back... let me add my 2 cents. Small boat prop selection is sort of a trial and error game IMO. Most boat manufactures don't do the necessary model (or full scale) testing do obtain the info needed for ideal prop selection. So what they do in most cases is try off the shelf props that are in the ballpark and see which one seems to be good enough without blowing the engine at wide open. There is actually a lot to it but since cost is the driving issue, pick one off the shelf and ship it! is the motto. You might say...how much would I gain if we had the perfect prop. You might do 10% better if you are lucky. To some that is a lot, to some...not. So off the shelf picks are the rule for pleasure boaters. Ships and competitive racing is another story. Anyway, you generally should pick the best prop (near peak eff.) at the design speed of the boat/engine, not at troll. If you decide you want a slower toll speed and you put a prop on that gives you that (reduce the pitch), you will over-rev the engine at high speeds. For most boaters, they think the "perfect" prop is one that lets the engine run at red line at wide-open throttle and they live with the performance at all other ranges. The hard thing about propellers is you only have one pitch (or pitch distribution)and that is only ideal at one advance ratio (inflow speed in essence) BUT you have a range of operational conditions, no single pitch and be ideal at all rpms and load. (That is why some ships have controllable pitch props..To match the pitch to the condition. ANYWAY, If you want a slower troll I would leave the prop alone if it were performing OK at other speeds. (The most important speed is the cruise condition) Can you ...Lower the RPM of the engine at troll?...(It sounds like you have a 4 cycle they are much better at low RPM then 2 cycles and you might be able to adjust the low speed throttle a bit to give you a slower troll) Also, you might think about creating drag some how...(toss over a sea anchor) Not very elegant but it would slow you down. If you troll a lot, you might want to consider a cheap trolling motor or (kicker), they can give you a great troll, the prop is so mismatched for the boat that you can creep along. If you have ever been on some of the newer 50 or 60 footers with big high torque diesels, they tend to troll way too fast because they need to have a big prop to absorb the power...have you ever been on some of these boats when they put them in gear and they lurch forward? A couple of the newer boats I have been on have a cool option...it is called a trolling throttle, it lets the transmission slip at low speeds to essentially let you get any trolling speed you want without touching the main fuel throttle. Sorry for all the babble, I would try and adjust the low speed throttle and not #^&#^&#^&#^& with the prop first.