"what pitch" ... again!

DonMacN · 13 April 2009, 20:10

Folks,

Sorry to re-hash such a well discussed topic but....

Just got the new engine on the boat today. Haven't used it yet because it came with a left hand rather than right hand control box/throttle lever.

I'm very much a right-handed guy, so will need to wait for the swap.

So, the prop that was ordered for the engine by the shop was a 13 x 13.75.

It's a 5.5m northcraft, weighs about 750kg, and will be used for a mix of diving and family cruising. THe engine is a Honda BF90 4-stroke.

All the props I've come across before - diving Club's 5m Humber/Yam 60; a mate's Delta 5.5/Yam 100 etc have been 19" or thereabouts. It was a 19" on the old Mariner that's just come off the boat, so 13.75 seems a long ways different.

I figure now's the time to swap it, as it hasn't been used, and wonder what would be the best to start off with. 19" again??

Thanks in advance.

D

Pete7 · 13 April 2009, 21:02

Donald, how does the gearbox ratio compare with the other engines, but yes seems a little low. If you use it will they exchange it if it's wrong? bet there is a bit of shuffling feet when you look them straight in the eye and ask.

Pete

DonMacN · 13 April 2009, 21:21

Pete,

Don't know about the gear ratios. I'm reading the engine manual as 'bedtime reading' but can't find any reference to that yet. There's nothing in the specification table.

I think I should probably be able to swap it OK, seeing as I had no input into the choice of pitch and it's a big enough outfit. I'll also ask the mechanic to phone Honda's 'tech support' tomorrow and see what they say. I have discovered that optimum throttle will be at 5,300 to 6,300 revs.

Nos4r2 · 13 April 2009, 21:47

Judging from the revs MHB100's Zodiac Pro Open was doing with the same engine, I'd try starting with either a 19"x13.75 or a 17"x13.75.
That's assuming the same gear ratio as the earlier Honda 90s.

Poly · 13 April 2009, 21:52

Don - do the Honda controls not allow you to reconfigure as either LHS or RHS the way that Yam ones do? I thought that was standard on most of the lower end controls...

A quick look on the web suggests your gear ratio is prob 2.33, other brands seem to vary from about this down to 2.0 in the 90HP size. So your prop may be turning up to 15% slower for the same engine revs.

Chris · 14 April 2009, 06:51

With a 2.33 gear ratio I would start at 19"

Downhilldai · 14 April 2009, 10:36

Quote:

Originally Posted by Chris

With a 2.33 gear ratio I would start at 19"

So would I.

My Northcraft 5.5 is just about right with a 17" pitch on a DT100 Soozook, which has a 2.08:1 gear ratio. This would be roughly equivalent to a 19" pitch on the Honda 90. If you're loading her up with divers & kit, then you may need the next finer pitch size, which would probably be 17"

Bigmuz7 · 14 April 2009, 17:11

I think mines a 19x13 on a year old BF 90, which without much load on a 5.7. I'll only get 5700 trimmed to the limit on the flat. If you are loaded you will need to go down, as the other guys have said .. I tried a 21 x 13.25 and had to laugh at the result .. I couldnt get it over half revs

These motors seem easily choked down at the bottom end of the rev range possibly because of the ratio and if anything I've come to the conclusion that I'd prefer to be slightly under propped, .. I'm away from the boat atm , and will confirm in a day or so that it is the 19

DonMacN · 14 April 2009, 22:30

guys, thanks very much for all the reponses. I'm sure it's not the 'black art' it appears, but I've never managed to find the time (and never needed to until now) to research the combinations of gear ratios and prop pitch.

I've asked for a 19" and it seems this won't be a problem as long as the one I currently have isn't used. SHould hopefully be up by the end of the week as I'd like to start getting the engine gently run in. There's a few dive trips starting to appear in the calendar and I'd rather not be restricted to 2000 revs!

As far as the controls are concerned it appears they are swappable in that a LH can be reconfigured to be a RH, but surprisingly (to me at least) the cover plate with the Honda branding etc will then be mounted in against the console, with a less protected face to the elements. I phoned Honda UK myself about it today because I just couldn't believe a company of that size hadn't made a 'proper' RH control. Seems they're considering it, but not really likely any time very soon. I'll just need to get out the sealant gun and ensure the box is as protected as it can be.

thanks again

9D280 · 15 April 2009, 11:43

Quote:

Originally Posted by DonMacN

As far as the controls are concerned it appears they are swappable in that a LH can be reconfigured to be a RH, but surprisingly (to me at least) the cover plate with the Honda branding etc will then be mounted in against the console, with a less protected face to the elements. I phoned Honda UK myself about it today because I just couldn't believe a company of that size hadn't made a 'proper' RH control.

Nobody does! (well, if they do I've yet to find them). Had Suzuki, Yam & Merc &a borrowed Tohatsu box - all the same story! The only real issue you'l lhave is showing off whose box you have - the hub of the lever usually covers most of the "ugly" bits!

The only truly "cosmetically reversible" one I've seen was an old teleflex thing from about 1970 ish. Only truly reversible because the lever would bolt on at any increment of 90 degrees, so for a "RH" you just spun the lever round 180 deg & mounted it upside down! (no fast idle levers (or neutral catch!) on that box.........

Suffice to say the deadman for that control was console mounted!

DonMacN · 16 April 2009, 20:34

yep. I'd never paid much close attention to the box on the Club boat - a Yam one, but when you do, it's obvious enough that the same thing's been done there, so Honda's not 'out of step' with the others. Still a bit surprising though.

It's switched over now, and doesn't look too bad.

cheers