First time out, ran into some plane and comfort issues

[HDAV]

18" I think is standard shaft not short, I think you will find transom to be 14" ish in which case a 15" short shaft would probably be better. Only way to know is to measure the transom (or get manufacturer data sheet) and try easing motor with some timber spacers.

[office888]

With standard boats, the cav plate should be ~0.5-1" above the keel.

Futuras are a little more different to set up, but that's another boat with another story entirely.

[lightning]

Also my boat tended to bang about and the floor would creak and flex on even slightly choppy water, spoiling any enjoyment of going fast.

As already mentioned, it was my tube pressures. I was using a RULE electric pump which did seem to blow the tubes up pretty well.

But after buying the Bravo pump (which also has a compressor) l found that the RULE was only inflating the boat to around 1 psi.

I now inflate it to 2.4psi (max is 3.4) and it's a different boat, still a SIB of course but much better if there are a few waves, and no creaking/flexing of the floor sections.

[jeffstevens763@g]

Pump it up to 3.4 and leave the dust caps off in hot weather as a relief valve just keep and eye on them to stop them going too soft

[simonafloat]

Originally Posted by jwalker

I think 15hp is not enough. Sibs are not efficient hulls and have a lot of drag nor are they very directional because they are flexible and the keel line is very rounded, also, the hull will turn into a gull wing section because it's simply a piece of fabric being pushed upwards by the water. Because the hull pushes up it gives two other characteristics; Number one, as well as the gull wing shape it forms a bit of a tunnel hull between the hull fabric joint and the tubes and secondly, in a side on view the hull fabric is flexed upward but has to curve down to the transom and this forms a hooked hull shape at the stern. Plus, of course, any wave under the boat will effect the hull shape at that moment in time so, in effect, the hull is constantly changing shape under the various loads produced by the waves. That's why the equivalent sized small rib is so much better a hull than a sib. This doesn't mean you can't improve your boat but at a distance we can only guess at a solution for you.

4.2m sib, 15hp should easily plane it with at least two adults. ours jumps onto the plane with two adults and child and luggage.

[Jlomb436]

Thanks guys.

I'm going to try and take it out next weekend to try a couple of the tactics below. I have a feeling the motor is just too deep but I'm going to keep the fingers crossed and see what the results are. I had a blast otherwise.

Now only if I had the vehicle, space and money for a nice 20 foot fishing boat

[Jlomb436]

JUst for kicks I took a look at the prop. It's 3 blade stamped with 9 1/4 x 10 1/2. I don't see or can find any other information.

Do you think a lower pitch or a 4 blade prop would be helpful? Top speed is nice, but I'd rather get on/stay on speed if it only cost me a few mph. I was at 21, so 15-18 wot with a 4 blade etc would be fine b me.

[jyasaki]

Shorter prop would help you get out of the hole and up on plane quicker, at the expense of speed (like a lower gear in a car.) Have to watch that you don't over-rev the engine for too long if you run WOT. A tach would help ensure that doesn't happen.

jky

[Fenlander]

>>>Shorter prop would help you get out of the hole and up on plane quicker

As the standard dia for your Mariner's prop is 9 1/4" I think 10 1/2" will be the pitch.

The standard prop on most 15hp 2-strokes is 9" and on all the SIBs I've used with 15hp 2-strokes I've rarely ever needed a larger pitch than 9".

Given your Bombard 420 is a heavy alloy floor version that weighs 181lbs the last thing it needs is a prop with higher than standard pitch.

Not only is there its weight but it will have a very large floor area pushing a huge wall of water just off the plane and a 10 1/2" prop will tend to bog the motor down as it tries to transition onto the plane.

Your current prop pitch will also tend to produce the effect you describe of it easily falling off the plane and then taking ages to get back up.

I've experienced similar recently with a new 20hp that was supplied with an 11" prop... I guessed it was too large a pitch and a drop to 10" has greatly improved things.

[Jlomb436]

Originally Posted by Fenlander

>>>Shorter prop would help you get out of the hole and up on plane quicker

As the standard dia for your Mariner's prop is 9 1/4" I think 10 1/2" will be the pitch.

The standard prop on most 15hp 2-strokes is 9" and on all the SIBs I've used with 15hp 2-strokes I've rarely ever needed a larger pitch than 9".

Given your Bombard 420 is a heavy alloy floor version that weighs 181lbs the last thing it needs is a prop with higher than standard pitch.

Not only is there its weight but it will have a very large floor area pushing a huge wall of water just off the plane and a 10 1/2" prop will tend to bog the motor down as it tries to transition onto the plane.

Your current prop pitch will also tend to produce the effect you describe of it easily falling off the plane and then taking ages to get back up.

I've experienced similar recently with a new 20hp that was supplied with an 11" prop... I guessed it was too large a pitch and a drop to 10" has greatly improved things.

Hi Fenlander.

Excuse my novice, but are you suggesting a lower pitch as I suggested? Thus reducing my top speed and getting out of the hole faster? Or do you think a 4 blade would suffice?

You mentioned my scenario and that it falls off plane. This is frustrating and caused me to bring the boat in early.

[Fenlander]

Yes I think a lower pitch shold help your situation... but of course it's only in trying another prop that you'll know for sure.

The theory of a 4-blade is that it should help in the midrange where you need the grip to not fall off the plane so much and get back on quickly when you do.

The other accepted theory of a 4-blade is that you usually drop a pitch down from the 3-blade that would be correct for you.

So if a 9" 3-blade would have been a sensible option then in theory a 4-blade should be an 8".

Be interested to hear from others about this as I know the theory but have never used 4-blade props.

[jeffstevens763@g]

what hes saying is coming down to 9 inch pitch is like coming down a gear with your current prop the engine cant cope with hitting a wave and staying on the plane plus it will get on the plane quicker you will loose a bit of top end speed but that will be a small price to pay with the the performance you will get and a more consistent speed conditions allowing!

[beerbelly]

he might not lose any top speed if the higher pitch prop is not allowing full revs at wot. I had a fletcher arrowflash 15 with a mercury 75 the po had fitted a 21 pitch stainless prop believing it would give him more speed but it would barely pull my mrs out on two ski,s and flat out it was just touching 45 but then started porpoising badly so you had to have it trimmed all the way in and couldn't hold top speed .I changed to a 17 pitch and it pulled me mrs out on one ski easily and if I trimmed the motor out a bit top end rose to 48

[jeffstevens763@g]

Beerbelly is right if not achieving WOT you might not loose top speed but I guess in the conditions you had WOT wasn't achievable anyway for any length of time
Reading your original post if the waves were significant enough you would do well to stay in a straight line at WOT and bouncing off waves would be scary.
When you say WOT what was you RPM

[Jlomb436]

Woa guys, appreciate all of the good feedback.






So would you suggest a 3 blade 9 pitch, or a 4 blade 9 pitch?




I honestly don't know what RPM I was running at. That's probably worth buying a small tach to see to ensure I don't over rev. Would you suggest I make the call on the pitch and prop once I know what RPM I'm running at? Thanks again.

[Poly]

It's almost impossible to optimise the prop selection properly without knowing what the rpm is.

[Fenlander]

>>> a 3 blade 9 pitch, or a 4 blade 9 pitch?

Diffficult call... we can all guess which we might choose but only trying on your boat with your motor and your load will prove which will do the job.

In many ways the 9" 3 blade is the safest option as that's what would have been on that motor as standard in the UK and will give good all round performance on many planing boats.

As I said before in theory to get to the same place with a 4 blade you need to pitch down to an 8".

I see props are cheaper in the USA than here. This looks the sort of thing you need... Solas Prop 1111-093-09 Mercury (48-82815) 9.25'' 9'' Pitch 3 Blade RT 9-15hp | eBay

Obviously check it is the right fit for your motor before ordering.

Edit: Poly is right of course tach readings would be very useful but I'm pretty sure a 9" pitch 3 blade would be a far better starting point than where you are now.

[jeffstevens763@g]

fenlander & poly are absolutely right but i would have a run out in calm conditions with the weight you are going to carry first with a tacho that will be your bench mark, me personally do quite a bit of trialing before buying my second prop.just bear in mind if the conditions are rough you will be off the plane more than on and for safety and comfort it sometimes better at displacement speed anyway.

cheers

[Landlockedpirate]

Ive played around with all sorts of props, stainless and ali, 3,4 and 5 blade. In the end they all end up on ebay (even the £500 High Five )

My advice is stick with the original diameter and number of blades, unless you have a very specialized set up (you havent) . Play around with the pitch, as said above, dropping down an inch or so will give you better control over the initial stages of planing at the cost of top speed.