Fitting outboard - centreline offset?

[rbcoomer]

Hi All,

I'm in the process of swapping over the outboard on our 14' Fletcher ArrowFlyte and wondered if there was any science in determining optimal offset position?

The existing Evinrude 30 hp is slightly offset to starboard (around 3/4") and as a rule the boat handles well. We get the inevitable 'list', but once 'balanced' by moving a few peeps/grub etc about it's ok. I appreciate that trimming a vessel this size if more about distribution of weight, but as the 'new' outboard is heavier (50 hp Evinrude), I don't want to risk making this worse or even destabilise the vessel! If I understand correctly, the offset is to help counter prop-steer and in an ideal world I'd fit, do a few high speed runs, adjust, re-test etc. The downside is obviously hole drilling in the transom and as I'm already adding two more 'filled' holes to half-a-dozen others and drilling 6 more, I want to avoid turning the stern into honeycomb

My current plan is to mimic the previous offset and rely on a once only fit. I'm planning on 6 x M8 fixings (SS) distributed over available holes in mounting bracket to secure the motor. Unlike the 30 which had just 2 x M8 and 2 'clamps', this unit has no 'clamp' fixings and there is no significant overlap into the splashwell. My thinking is that prop-steer is going to be a torque thing and proportional to size of prop, HP etc. As this is a bigger prop and more HP it would suggest I don't really want to have a lesser offset. Because of weight and stability however, I'm not sure I should add any extra offset either?

Any thoughts and advice from the Sages here greatly welcomed!

TIA

Robin

 

[jfm]

I would think 3/4 inch offset to stbd is fine (for a r/h prop, which surely you will have). I've always done mine 3/4 inch with no probs. It is not an exact science until you get to MapisM-style uber high performance craft

M8 seems very weedy but if that is the size of the holes in the engine brackets then so be it. I'd much prefer M10. With M8 make sure to seal well and get the sealant right down into the hole so it "paints" the bolt, esp if the transom is damp plywood. This to resist crevice corrosion which turns M8 into M5.

 

[rbcoomer]

Thanks JFM - 3/4" it is then!

TBH the holes probably will take bigger - I'll measure tomorrow. The bolts that came off the 30 were M8, but bigger certainly can't hurt if they'll fit the holes...

I think the transom's dry, but take your point and will seal generously

 

[Kawasaki]

As jfm 3/4 is ok but do use 10mm
Does the 50 have power trim?

 

[rbcoomer]

Will measure the holes and get largest I can fit!

Sadly no, not PTT - I looked around but all were going for more than I could justify at the moment. This 50 cost just £500, but all the PTT's were around three times that. I'm trying to conserve the 'boat budget' for the Arrowbolt project which has already stalled due to weather and other priorities! The upgrade was more for reliability and a bit of extra speed to get us past the 22kt barrier...

 

[rbcoomer]

Thanks for the feedback - 10mm is a much better fit. I've only done the top 4 so far as I forgot to check before I went out yesterday and thus bought the bolts etc 'on spec' on my way home. The top 4 go through into the splash-well however and once I've pulled the tank out I figured I can do 4 more lower down rather than 2, so will do those next weekend. Added generous quantities of Sikaflex to each hole too. Run up and tested ok - she starts on the button and healthy water output, so now just need finish up and await a decent day for a trial run.

I'm struggling on the model number if anyone has any tips? The plate says model is E50RNL TA but this doesn't seem to conform to the numbering conventions I can find. The nearest match I can find is E50RNLIB here http://www.boats.net/parts/search/BRP/EVINRUDE/1992/E50RNLIB/parts.html - and in fact all the parts diagrams seem to match up, so circa 1992 seems best estimate so far. I was a bit concerned about the 'N' which various guides suggest means 'counter rotation', but prop is clearly RH... It's electric start which seems to make a mockery of the 'R' as well, although that could obviously mean an electric starter has been added.

Edit: Ignore last bit - seems the 'T' must be an 'I' so model is in fact an E50RNLIA and thus 1990. Now just need to find a power tilt unit (0397532 or 0438135) at a sensible cost!