Merc 15 efi - Merc 20 efi Performance Difference.

[Steve509926]

So I finally got myself a "new to me" Merc 20hp EFI, 2 years old, just over 10 hrs on the clock.

You may wonder why I didn't get a 20hp when I bought Redneck, an Excel SD360, just over 2 years ago, it's very simple, there wasn't any new 20hp EFI's available at the time.

I managed to get out on the water yesterday to test it and see what difference 5hp makes to Rednecks performance. Set up as I usually do, 2 up + dog, 17 litres of fuel, usual kit for a day.

These figures are without a tiny tach (yet another broken one) so I couldn't check wot rpm for the 20hp.

15hp @ wot 16.2kn
20hp @ wot 18.4kn

Pleased with the extra 2.2 knots, but the following figures pleased me even more.

20hp 1/4 throttle 10 knots
20hp 1/2 throttle 14 knots
20hp 3/4 throttle 16 knots

I like to cruise anywhere between 12 - 15 knots, so the fact that I can now do that speed a little over 1/2 throttle means a saving on fuel, less wear on the OB, but a really noticable difference was how much quieter it is.

As you would expect acceleration and getting on the plane was much quicker.

I'll check rpm when another tiny tach arrives, but I'm not expecting it to be out of mid range.

There is absolutely nothing wrong with the 15hp Merc EFI, I fact I think it's a great OB and if that's the max your sib can take I would highly recommend it.
But, if your sib can take a 20hp and you can afford the extra, go for a 20hp.

[chipko]

What prop pitch are you running…15 and 20??

[Steve509926]

Hi Chipko,

15 - 8p - 6160 rpm - 16.2 knts

I had tried a 9p but that ran at 5725rpm - 15.6 knts

The 20 has a 10p on it but I don't know the rpm.

[Fenlander]

Always like a bit of real life test data Steve. Congrats on the new motor.

Life is still effin rubbish hence I have nothing new SIB wise to offer so excuse some rehashing of my old data as a comparison.

When I had the 20hp Suzuki I found its greatest virtues were near silent trolling and more important the ability to get back on the plane after easing back in difficult sea conditions working the waves. Oh yes and the ability to handle a third adult in the SIB without knocking performance too much. 10P was for me the ideal pitch on the 20 with two adults, dog and kit plus it suited too when the third person was added. We were usually around the 20kts mark flat out once the 20 was fully run in.

My Yamaha 15hp 2-stroke made 17.9kts with the prop that suited best... a 9.75P

Our Tohatsu 9.8 2-stroke manages 15kts two up on its 8.5P prop but is only snappy on and off the plane one up.... bit more sluggish two up. A 12kt cruise is sensible for this motor and allows the throttle to be backed off. As a matter of interest it managed 17kts on the Elling.

Another random fact is that my Tohatsu 9.8 2-stroke has the same fuel consumption as the Suzuki 20efi when both are on a fast cruise... the 20 of course travelling about 3kts faster.

[Steve509926]

No problem rehashing old data David, I didn't know that props are available in 1/2 & 3/4 pitches, so all data, including old data, is useful.

The new tiny tach arrived yesterday so I will be able to check rpm on the 20hp, probably not until next year, but you never know

It was a shame you and Mrs F didn't find the Elling suitable, that's impressive with a 9.8.

[Highland Haggis]

Glad your enjoying your new engine Steve I have got no further with a new boat or engine not sure what I am doing yet but will look at something soon all the best HH

[Steve509926]

Cheers Haggis

Hope you get something sorted.
Did you get anywhere with the police and the insurance?

[Brinormeg]

Hi Steve. With your..... figures perhaps you were right to think the Volaire would give you a bit of a boost if your 20 is the same as the Suzuki 20.

I'm guessing that the inflatable keel design helps with the top speed as well as getting on the plane quite quickly.

I believe I was getting approx 23mph with the factory 12" prop at 5200/5300 WOT

PD says he went to the 10" prop which he found to give sharper control and a bit quicker with 6000rpm wot

Never really got a chance to try it at different throttles but guessing you would see a good bit of improvement with my set up.

After destroying our tac I bought the very cheapeset one I could find along with another different design with a step up in price.
Fitted both.....one on each lead, and kept one under the cowl and one outside.

Figure we cant break both

[Steve509926]

Hi Brian, I don't know the rpm range for the Suzi 20 efi, but I would have thought 5200 was at the lower end.
I think I'm at a cross roads in life. I would like a little more space, as I know we will soon be more often than not 3 up + dog or 2 + 2 kids + dog, hence the hankering for a Volaire 390. However I also know that the weight of the deflated SD360 is at my upper limit to lift solo. I need to think about the Volaire 390 before I take the plunge and think very carefully how I would move such a heavy lump around.

[Brinormeg]

Originally Posted by Steve509926

Hi Brian, I don't know the rpm range for the Suzi 20 efi, but I would have thought 5200 was at the lower end.
I think I'm at a cross roads in life. I would like a little more space, as I know we will soon be more often than not 3 up + dog or 2 + 2 kids + dog, hence the hankering for a Volaire 390. However I also know that the weight of the deflated SD360 is at my upper limit to lift solo. I need to think about the Volaire 390 before I take the plunge and think very carefully how I would move such a heavy lump around.

5300-6300 range in the owners manual if I remember correctly (but of course my tac could be slightly out), but I will know better when i plug the OB in to the pc.

Sib wise I will stick with what I have, as dropped quite a lot of money into it this setup already, but my next purchase will be an extra prop (just in case). Going by davids and pd experience it would be the 10. But will let you know how we get on when the time comes.

So far we have managed the Volaire, but that was setting up next to the car almost everytime, and just wheeling it to the dock/slip etc (which I can do single handed if need be even fully loaded).
Setup is usually with my teeneage son and the one time we had to lift the boat 30 yards or so in its bag, we did struggle.
This time at Loch Awe it was just the better half and I, but again set up right next to the car, and quite shallow slope to haul the bugger back out.

I wasnt really fit to go from shed to car or to move it about for washing on my own, but have now trialed a board with wheels, which on smooth ground makes it ok...ish.
But to be honest it isn't so much the weight but the actual bulk...especially without a decent bag, when with one slight drag instead of lift we did damaged the sib a bit.
A sack barrow could be an option, but i think the pure size would make it difficult to use with one unless you adpated it a bit.

So I guess it depends on launching sites and who you are going to have with you.

But of course you could just have 2 sibs

[Steve509926]

Originally Posted by Brinormeg

But of course you could just have 2 sibs

So Diane's number is 078.............., you're probably in a better position to explain the rational behind that thought

[Brinormeg]

Originally Posted by Steve509926

So Diane's number is 078.............., you're probably in a better position to explain the rational behind that thought

Yip......about 500 miles north

[Steve509926]

Originally Posted by chipko

What prop pitch are you running…15 and 20??

Hi Chipko, what prop pitch are you using on your Merc 20 efi and what sib are you using it on?

[jeffstevens763@g]

Nice one Steve, in my experience with sibs 5 hp gives 2 knots more top end depending on prop and load but it will allow you to prop down if you need a crisper response in the rough if you need to slow for a decent wave and get back on the plane quickly.

[Steve509926]

Originally Posted by jeffstevens763@g

Nice one Steve, in my experience with sibs 5 hp gives 2 knots more top end depending on prop and load but it will allow you to prop down if you need a crisper response in the rough if you need to slow for a decent wave and get back on the plane quickly.

So - 15hp @ wot 16.2kn, 20hp @ wot 18.4kn - sounds about right.
Hopefully we'll get a bit of decent calm weather before winter sets in. I wouldn't mind getting over to Anglesey to check rpm especially whilst I can still use the 9p prop from the 15hp before I sell it.

[jeffstevens763@g]

With all my kit for camping running 10/11/12 inch pitches there’s about 2 knots difference at WOT but the 10 is my favourite much crisper

[Steve509926]

Thanks for the info Jeff, very helpful.

[chipko]

Originally Posted by Steve509926

Hi Chipko, what prop pitch are you using on your Merc 20 efi and what sib are you using it on?

Hi Steve, congrats on the new motor.

Running a 10” pitch prop on our 20 and when on the T38 good for 21 knots one up plus kit. Currently powering our tiny but heavier Ocean Runner 340 rib which maxes out at slightly over 23 knots on the same prop.

For our use have found the 10” pitch gives us decent top end speed whilst maintaining snappy hole shot and useful mid range grunt. We also found the same pitch suited our Suzuki 20 (efi) on Zodiac and Aerotec sibs which often towed our teenagers on toys/wakeboards. Also good for 21 knots solo plus kit.

Thanks for comparison data between your 15 and 20. I’ve probably wrongly assumed a 15 may be a knot or so down on top end speed but comparable in the mid range due to same block developing similar torque.

Out of interest are your 8 and 9” props Mercury or aftermarket brands?

[Steve509926]

Thanks Chipko.

I've always thought that the SD360 isn't the most efficient "hull" design on terms of speed, with it having a sausage keel and being so wide in the water (benefit is having more internal space) and your figures tend to confirmed that.

The 9p is OE
The 8p aftermarket.

It'll be interesting to compare the aftermarket 10p I have with the OE 10p fitted to the 20hp.

[chipko]

Whilst I’ve nothing against aftermarket props per se, and am sure some will be better than many OEM, but found pitch/performance inconsistent between makes.

Must admit when testing for optimum pitch size prefer to stick with same type/make for an accurate comparison.

Good luck with the testing. [emoji106]