Help identifying my prop

Daddy Matt · 18 July 2019, 10:43

Hi guys

New boy wanting advice.

I've got a Tohatsu M25C3 it's stamped as a 1999 model.

I've just changed the carb gasket and adjusted timing to bring it up to the 30hp.

I'm in need of replacing the well worn and chipped prop.

The stamp on the prop is heavily over painted and I'm not sure if I'm reading it right.

13 3/4 ? Thats what it looks like to my untrained unskilled eyes.

Can anyone throw some light on the situation for me?

Thanks

Daddy Matt · 18 July 2019, 11:07

Actually it could read 11 1/4?

Orwell boy · 18 July 2019, 11:16

reckon it should read something like (not yours just an example ) mine reads
3x11 5/8" x 13 say
3 blades 11 5/8 " diameter
13 pitch (which roughly translates to how far forward the boat will move forward for one rotation)

Fenlander · 18 July 2019, 11:18

To get an accurate idea I'd wire brush that stamp to be sure. It may be an aftermarket prop so not have the OE type numbers you'd be expecting to see.

Here is a prop table for a slightly later Tohatsu 25hp 2-stroke, the 25H but I'm guessing the fitment range will be similar.

You'll be able to check the dia is correct as it should be just under 10" but the pitch will need to be deduced from the stamp.

Fenlander · 18 July 2019, 11:24

Also see this parts list for a 2002 year motor of your model. The different props as shown in the side list under item 10. It mentions the 13" pitch as standard.

https://en.impex-jp.com/catalogs/out...pump-5738.html

Daddy Matt · 18 July 2019, 11:57

All that's readable after a wire brush is 11 1/4

Theres nothing else readable.

Just waiting on a reply from Tohatsu help line as to the genuine size of the 25hp
and if there's a difference for the 30hp that it is now.

Fenlander · 18 July 2019, 12:21

If you change that parts listing to a 30hp version it still shows a 13" as the standard pitch.

Are you looking to change the performance or is the current prop OK apart from being tatty? If the latter you could sent it for a refurb at about 60% of the cost of a new one and they would tell you its pitch. Then if you wanted to make future changes you'd start from a known position.

Daddy Matt · 18 July 2019, 13:29

I've just had a reply from Tohatsu

The standard prop for the 25hp and 30hp is an 11" pitch??

That would maybe explain the 11 1/4 stamp.

Fenlander · 18 July 2019, 14:25

Well not really... Tohatsu didn't do props with pitch sizes in fractions for that HP range apart from the 14.2" which they called a 14" anyway.

And I'd reiterate my question if you are after a like for like replacement or wanting to make a change. The "standard" prop any OB comes with is just a start point and factors such as hull shape, load carried and intended use will then lead you often to a different pitch for best performance.

Daddy Matt · 18 July 2019, 16:03

Well I'll give up on trying to identify the unmarked prop then. I have read up a little on doing a refurb on the prop and I don't think it's worth the effort or wait. Too many pits and chips on the leading edges and one blade is warped.

So I'll open it up for advance with experience.

Daddy Matt · 18 July 2019, 16:07

The boat is a Zodiac YL380dl

Engine is Tohatsu M25C4 Long shaft.
I've changed the carb gasket and adjusted timing to unleash the full 30hp possible from that model.

Payload will be two medium adults and two small teenagers.

It will never tow skiers or toys and will mostly be beach hoping with an occasional up estuary run for a pub stop.

Any suggestions to comfort or speed results gratefully excepted

Fenlander · 18 July 2019, 16:34

The refurb props come back like new so I'd not worry about its current condition but I guess you might have a two week overall turnaround. Steel developments are the people I've used and they often have pre-refurbed props in stock which would save the wait.

Weirdly I just wondered if they might have one to suit you are they are currently listing an 11 1/4" pitch for a Tohatsu (assuming you have a 3" gearcase)… plus a 13" too.

https://www.steeldevelopments.net/re...x-18-30hp.html

To get the best from a prop pitch choice you need a start point of OB revs vs measured speeds with a known pitch. I would guestimate for a SIB with 10-20hp motor but your small RIB is outside my experience.

Daddy Matt · 18 July 2019, 17:43

Strange

Daddy Matt · 18 July 2019, 17:49

Wish I had known earlier.

I've just bought an original at £100

Could have got 11 1/4 and the 13", and had change, and something do do a comparison with.

carldalmas · 19 July 2019, 01:45

Quote:

Originally Posted by Daddy Matt

Wish I had known earlier.

I've just bought an original at £100

Could have got 11 1/4 and the 13", and had change, and something do do a comparison with.

Check cav plate is not too low! on transom when engine is down ! As long shaft ??? It should be just a little higher than bottom off boat !
At WOT there should be no splashes and a clear view off cav plate out the water at WOT for max performace !
If engine is two low fit wood to top off transom to higher O/B to correct height by wood thickness

Fenlander · 19 July 2019, 10:26

>>>I've just bought an original at £100

Did you go for the 11"? Assuming so hopefully that will suit but if not did you know you can have your old prop re-pitched at the same time as re-furbing.

Daddy Matt · 19 July 2019, 15:11

I've bought an original, which I'm told is 11 1/4 as the stamp says.

When I have time I may look into getting the old one refurbed and maybe get them to adjust it to 12" pitch if its not to far gone for that.

I guess time and testing will tell all.

I've only had the boat a few weeks and not really had time to put her through her paces.

That'll change in a fortnight as we're off down to Cornwall for the holiday.

Staying just up the road from Fowey so plenty of opportunity for proper testing.

Fenlander · 19 July 2019, 15:17

Oh well enjoy Fowey... we loved our two weeks there last year.

Locozodiac · 21 July 2019, 15:17

To get the max out of your Rib/Motor combo will suggest you buy an induction tach say a Hardline Hour/Tach induction tachometer model : HR-8061-2 for a 2 cylinder motor. Check Ebay, Amazon for them

Install it at front of cowl, with usual deck load well balanced, motor trimmed to 90º (fully vertical) go for a wot run on flat calm no wind water cond if possible and check which was the max rpm achieved.

The Tohatsu 30 HP motor has a wot range of 5200-5800. Ideal is for motor to run middle 5500 to max 5800 rpm as usually loaded, if with a fixed load much better. With a OEM 11 pitch prop motor should run at least 5500 wot rpm, only a water test will tell.

Those 30's likes to be transom height seated for water flow to skim right under the small upper plate, right under the protruding edge. Will be good that you check at which lower leg is water flow passing by at full throttle/plane as loaded.

If water flow passes slight over the upper small plate, will achieve water splashes side or even over motor.

If water flow skims at middle of both plates or very near AV plate will achieve prop aeration on choppy, wavy, windy water cond and at tight close turns at speed. The motor sits too high on transom.

This is the ideal motor/transom height for your combo, best prop thrust at all water cond, tight close turns and excellent top end speed.

Happy Boating