Prop choice

[codprawn]

My new boat 8.8m 225hp 2 stroke in case anyone forgets.....

Currently there is a 4 blade 19" pitch stainless prop fitted - should be great for acceleration and carrying 12 people but prob won't give much of a top speed.

Have just bough a lovely suzuki 24" stainless prop off ebay - 3 blades. May be a bit too fine a pitch though - to complicate matters there is a transom jack.

Just wondering if the 24" prop is going to be suitable - even empty the boat will have 500l of petrol on board. I suppose it's just a case of try it and see???

[Alan]

Codders you know prop selection is a bit of a black art so 'suck it and see' is a good approach. However, gut feel from me would be that 24" would be too coarse a pitch and that you wouldnt be able to pull the right RPM's and would be straining the engine. IIRC Blue Ice (7.5m/200hp) runs either a 21 or 23" prop - I think Richard went to the 21" after experimentation. I am not sure how much difference the jackplate will make (ask Jonny F or Cookee as thats more in their area of h'expertise), but as I say I reckon 24" is too high. Your 19" will get you from CI to the mainland - a trip where you will be fairly loaded down with cheap fuel yes?

[ADS]

Codders, I wouldn't worry to much about it until you get to try it, I assume you will be running it in on the way home so there won't be much need for flat out.
Once its run in you need to see what revs its pulling at WOT and check that its within Suzuki's recommended range (5400-5600 I think on my engine), yours should be in you Suzuki manual you bought of ebay!
Best thing to do is test it light and loaded, but because your boat is so unique with the transom jack etc, nobody can probably tell you.
You may wish to have two props one for when your carrying people and one for when your light, to test loaded you could fill up a few drums of waters and see how she performs or just find 12 people, count me and Hugh in
You have two props to play with and a decent propeller company (steel developments, propeller revolutions) could repitch either of those.

Alex

[roycruse]

Originally Posted by codprawn

Currently there is a 4 blade 19" pitch stainless prop fitted - should be great for acceleration and carrying 12 people but prob won't give much of a top speed.

I work out your max theoretical speed with that prop to be 47 knots - actual speed will probably be in the low 40's

but it sounds to me like you are carrying a lot of weight and you may find going up any higher pitch you will struggle to reach max rpm.

what does your boat weigh without the fuel - and will you always be carrying 500kgs of fuel around - i would try to only fill her up when you have a long trip to do its pointless burning fuel just to carry fuel...

[codprawn]

I thought 24" was a finer pitch than 19" or - have I got it the wrong way around?

[roycruse]

IMO coarse and fine are the wrong words to describe the pitch of a prop - long and short are far more descriptive or steep and shallow

you would describe a thread on a bolt as course or fine - so if you do use those words then a 24" prop would definitely be coarser than a 19" as a coarser thread pulls a bolt further per revolution so a coarser prop would pull a boat further per revolution

[codprawn]

No wonder I get confused - as you say much easier with a bolt - 4 tpi would be really coarse and 240tpi would be really fine.

[simmons0]

Originally Posted by codprawn

I thought 24" was a finer pitch than 19" or - have I got it the wrong way around?

24" is a larger pitch than a 19"

A 24" prop would move 24" inches through the water in one revolution wheras a 19" would only move forward 19" in one revolution.
At least thats my understanding.

With a lifter i'd guess you could maybe go up an inch of pitch over a similar boat without the lifter.

Harry

[DepSol]

41knots to be exact.

Dom

[jyasaki]

Originally Posted by codprawn

Just wondering if the 24" prop is going to be suitable - even empty the boat will have 500l of petrol on board. I suppose it's just a case of try it and see???

Congrats on the new arrival, before I forget.

19" to 24" is quite a change. General rule of thumb is 200 to 250 rpm for each inch of pitch change (down in pitch = up in rpm.)

What I found in my limited testing is that the rule of thumb is an extremely rough estimation. Mine went from 19" pitch (turning about 4000 rpm) to a 13" (which should have put it just over 6000rpm) and couldn't get a reading as it sat on the rev limiter at about 6100.

Ended up with a Ally 15", turning a tad over 6000 with just me and a less-than-full fuel tank.

Now running a SS 15" which turns about 5800 with a light load, about 5500 to 5600 with a "normal" load.

So, whether or not the prop will work for you depends on a lot of things, not the least of which is where the 19 falls in the rpm range. The 24 should put you somewhere around 1000 to 1500 rpm higher, in addition to less of a hole shot, and a substantial increase in top speed (assuming you don't lug the motor down.)

jky

[jwalker]

I suspect 19" will be about right.

Using 5500rpm, gear ratio of 1.8, slip of 18% and 19" pitch you'll be doing 39knts.

On my 6.5mt with the 200 Suzi it pulled about 46-47kts.

So that calculation is going to be pretty close on your 8.8mtr unless it's very light. 24" will be way to high.

[codprawn]

I know but it was a good price!!!

[Cookee]

I am sure the 24 will be too big for that boat - the 19 will be fine to start with - maybe lightly loaded it would be ok with a 21, the type of prop also plays a big part as does the balance of the boat fore and aft. If you ask me I wouldn't buy any more stuff untill you've run it in and know wether you're getting anywhere near the rev limit or not.

If you go back to when you asked advice about the lifter I told you at the time not to get one, but Stainless Marine were the best - Regardless of that I have another little nugget for you - get a water pressure guage and watch it like a hawk or you WILL blow the engine up! Lifting the engine once you get going has 2 effects - it reduces drag on the gearbox which allows higher speeds, for instance at 65 mph you would expect 3-5 mph gain, and it also reduces bow lift as the engine gets higher.

The other thing that lifters and set back brackets do is put more stress on the transom (think of the leverage effect).

[Searider]

Originally Posted by codprawn

I know but it was a good price!!!

That's why it was a good price

Aren't the builders (or engine suppliers) going to trial the boat with various props?

Depsol said 41 knots exactly - does he know something we don't - being in the CIs?

[codprawn]

Originally Posted by Cookee

I am sure the 24 will be too big for that boat - the 19 will be fine to start with - maybe lightly loaded it would be ok with a 21, the type of prop also plays a big part as does the balance of the boat fore and aft. If you ask me I wouldn't buy any more stuff untill you've run it in and know wether you're getting anywhere near the rev limit or not.

If you go back to when you asked advice about the lifter I told you at the time not to get one, but Stainless Marine were the best - Regardless of that I have another little nugget for you - get a water pressure guage and watch it like a hawk or you WILL blow the engine up! Lifting the engine once you get going has 2 effects - it reduces drag on the gearbox which allows higher speeds, for instance at 65 mph you would expect 3-5 mph gain, and it also reduces bow lift as the engine gets higher.

The other thing that lifters and set back brackets do is put more stress on the transom (think of the leverage effect).

The MAIN reason I got a transom jack was to give me extra space in the boat - it has 10" of setback. The idea was to have a rear bench seat but that's another story......

Yes I have asked for a water pressure guage to be fitted which I will watch like a hawk - if it proves to be a prob will fit a low water pickup.

I know about the extra stess involved - which is why when I get the boat I will replace the washers with some hefty pieces of angle.

[codprawn]

Originally Posted by Searider

That's why it was a good price

Aren't the builders (or engine suppliers) going to trial the boat with various props?

Depsol said 41 knots exactly - does he know something we don't - being in the CIs?

Not the case - bloke had been using it on a 19' light boat so he never had probs.

Doubt it - I specced the 19" to be supplied for full load use - as Cookee says a 21" will be the other option but I thought I may as well try the 24" to see what happens.

As to what Depsol says I was wondering the same myself - they are supposed to inform me when they are doing the sea trials - they haven't mentioned anything!!!

[Hugh Jardon]

Originally Posted by codprawn

I thought 24" was a finer pitch than 19" or - have I got it the wrong way around?

with the size and beam of your boat do you have any idea of prop slip expected and what you think the max speed will be for your boat?

Not sure you will be able to pull a 24'' prop but it is trial and error.

[Jono]

Originally Posted by codprawn

Not the case - bloke had been using it on a 19' light boat so he never had probs........

I thought it was new "old stock" engine?

[Hugh Jardon]

this is getting interesting


seems some elements of the story are starting to unravel .................

[Hugh Jardon]

Originally Posted by Jono

I thought it was new "old stock" engine?

silly billy......

its a second hand new "old stock" new model from the 90's