Prop Blades, SIB and trolling

[Slayer33]

So, I'm looking at getting a lower pitched prop. I have a merc 20hp 4-stroke and a 12' SIB. Total weight is about 800 loaded up with gear.

The stock prop is a 9-3/4 x 10P.. I am considering getting a 8P in the same diameter... I need to slow my trolling speed down and figured that a 9P wouldn't make enough of a difference.

My option is either a 3 blade or a 4 blade prop in the 8 pitch.. Cost is almost the same, so that isn't a factor.. but can anyone enlighten me on which one to choose?

From what I've gathered 4 blades seem to have more positives than 3 blades.. but what are your experiences?

[tamsbairn]

Hi Slayer33 have a look on YouTube for a guy who has few video and is called "The propellor hub" some good prop tips and working out all to do with props , might help
Stewart

[andrew_ireland]

Originally Posted by Slayer33

So, I'm looking at getting a lower pitched prop. I have a merc 20hp 4-stroke and a 12' SIB. Total weight is about 800 loaded up with gear.

The stock prop is a 9-3/4 x 10P.. I am considering getting a 8P in the same diameter... I need to slow my trolling speed down and figured that a 9P wouldn't make enough of a difference.

My option is either a 3 blade or a 4 blade prop in the 8 pitch.. Cost is almost the same, so that isn't a factor.. but can anyone enlighten me on which one to choose?

From what I've gathered 4 blades seem to have more positives than 3 blades.. but what are your experiences?

4 blades will give better bite and acceleration at low speeds while 3 blade props will give a higher top speed, in my own opinion there's no difference between them at trolling speeds.
Regards

Andrew

[jonp]

There are other options too Happy Troller - Large | Davis Instruments. Also some trim tabs go in the fully down position to reduce speed, even non electric models have a lever to lock in the down position. So,e of the guys I know who need to reduce speed to troll for trout simply deploy a sea anchor close behind the boat.

[Fenlander]

Slayer do you remember when you first asked about this in another thread I told you I happened to have measured the tickover trolling speed of my Suzuki 20hp 4-stroke with a 10" standard prop.... it was 2.14mph.

Going through the calcs (given your OB is broadly the same in spec) swapping to a 9" prop might reduce that to 1.9mph... an insignificant reduction. Even an 8" would only drop you to 1.7mph.

I reckon to make a noticeable difference you'd need to drop to a 6" (if a pitch that small is made) for just over 1mph.... is that slow enough for you?

Of course a 6" will have a serious effect at higher speeds possibly knocking 10mph off the top speed and meaning it's revving its nuts off all the time when travelling from place to place.

If you are going to do so much very low speed trolling then something like jonp mentions or a second motor looks a better option... say elec or a 2hp 4-stroke.

[paintman]

Transom room for a second engine might be a problem.
On my current boat - 17' Dory - I have a 4hp 2-stroke Suzuki which sees use as back-up & for trolling as the main engine is simply too fast even at tickover for trolling.
I use a sea anchor to slow the boat down when drifting as being a planing hull wind pushes it along at a fair old rate. I've used them for years on various hired boats when trout fishing for the same reason & I'm inclined to think that one of those might be your best & probably cheapest option.

[Fenlander]

Yep compared with a new prop cost and the disadvantages at higher speeds the sea anchor is well worth trying.

[Slayer33]

Originally Posted by Fenlander

Slayer do you remember when you first asked about this in another thread I told you I happened to have measured the tickover trolling speed of my Suzuki 20hp 4-stroke with a 10" standard prop.... it was 2.14mph.

Going through the calcs (given your OB is broadly the same in spec) swapping to a 9" prop might reduce that to 1.9mph... an insignificant reduction. Even an 8" would only drop you to 1.7mph.

I reckon to make a noticeable difference you'd need to drop to a 6" (if a pitch that small is made) for just over 1mph.... is that slow enough for you?

Of course a 6" will have a serious effect at higher speeds possibly knocking 10mph off the top speed and meaning it's revving its nuts off all the time when travelling from place to place.

If you are going to do so much very low speed trolling then something like jonp mentions or a second motor looks a better option... say elec or a 2hp 4-stroke.

I understood that part where you explained the speed differences in pitch.. what I am looking to do is purchase a spare prop to carry with me at all times, and I have decided on an 8 pitch which may be my permanent prop and the stock 10P to be set aside as my spare.

I was more curious to know what the differences on the low and top end would be between a 3 vs 4 blade prop...

I've been told the trolling plates are not very good and to stay away from them.

[Fenlander]

The 3 vs 4 blade prop is a side issue as a 3 blade will cover pretty well all SIB use with great success.

4 blades will give better grip on the water and are often used for a heavy non-planning boat or their mid range grunt on a planning boat.

If anything I'd say there is a danger a 4 blade's extra grip could give a faster troll for any given pitch.

My personal feeling is still that an 8" will make such a small difference to trolling speed yet spoil your higher speed potential that the 20hp could otherwise have given.

[jonp]

As I mentioned earlier a set of tabs can be used to slow the boat down in the drop down position and also improve handling in the up position Smart Tabs Trolling Plates for Walleye Fishing Reduce Trolling Speed.

Watch the video