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Old 09 September 2019, 16:03   #1
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My boat don't plane

Hi everyone

I have a 2.5m airdeck Mercury Quicksilver ( 28kg ) and a 4stroke Tohatsu 5Hp ( 28kg ) short shaft

Can't get on plane

I have done this:

0. I have inflate all compartments at the prescribed pressure including the airdeck.

1. changed different trim positions even the first one closest to the transom

2. moved myself in different positions including towards the bow and even on top of the bow.

3. start very slowly and increased gradually the rev

4. start with a high rev

No matter what I do the engine digging in the water immersing the transom and raising the bow.

I give up as yesterday I have spent almost 2h on the sea and constantly doing adjustments but getting the same results.

If anyone can throw some advise would be much appreciated

Thank you advance guys for your time to read this and eventually for any future involvement in solving this unpleasant situation

Respectfully

Manus
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Old 09 September 2019, 16:29   #2
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Hi Manus and welcome to the forum.

Many small SIBs can plane one-up with no extra weight of kit using a 5hp but sometimes a SIB nearer 3m than 2.5m is actually better.

The only thing I can think of is... have you tested the pressures... floor in particular... after it's been sitting on the water 5 mins... is the outboard making its full revs... what is the prop pitch... and is the outboard shaft length correct for the transom?

David
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Old 09 September 2019, 17:07   #3
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Quote:
Originally Posted by Fenlander View Post
Hi Manus and welcome to the forum.

Many small SIBs can plane one-up with no extra weight of kit using a 5hp but sometimes a SIB nearer 3m than 2.5m is actually better.

The only thing I can think of is... have you tested the pressures... floor in particular... after it's been sitting on the water 5 mins... is the outboard making its full revs... what is the prop pitch... and is the outboard shaft length correct for the transom?

David
Thank you David for your message.
Yes the floor is it at his recommended pressure.
The outboard is brand new so is the boat and yes the engine is short shaft as recommended by the boat manufacturer, max 30kg and 5hp
I had another 2.3m inflatable before which went on plane at less that 50% of the rev using a smaller 3.5hp mariner
And yes is it only myself onboard with no any additional gear .
I知 so confused
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Old 09 September 2019, 23:00   #4
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Don稚 know much about SIBs but the chance of planing with 5hp must be very slim, it surely must be down to total weight at that level. What the OP hasn稚 told us is what weight is he/she because all due respect we could come up with suggestions forever and as an example if he/she is 20 stone it is never going to happen.
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Old 10 September 2019, 01:17   #5
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Yes good point 😀
I知 14 stones.
The idea is that I was going on plane with my previous inflatable using a 3.5hp 🙈
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Old 10 September 2019, 01:32   #6
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That's a horribly weightly lump on the back of a very short SIB - many of us use the 15hp Merc/Mariner 2 stroke and that is only another 8 kg or so for three times the power.

As others have said - short boat+ underpowered/overweight engine + poor weight distribution = a struggle!

Presuming the engine is performing correctly and not trimmed right out or something.
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Old 10 September 2019, 03:13   #7
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Quote:
Originally Posted by Manus View Post
Hi everyone



I have a 2.5m airdeck Mercury Quicksilver ( 28kg ) and a 4stroke Tohatsu 5Hp ( 28kg ) short shaft



Can't get on plane



I have done this:



0. I have inflate all compartments at the prescribed pressure including the airdeck.



1. changed different trim positions even the first one closest to the transom



2. moved myself in different positions including towards the bow and even on top of the bow.



3. start very slowly and increased gradually the rev



4. start with a high rev



No matter what I do the engine digging in the water immersing the transom and raising the bow.



I give up as yesterday I have spent almost 2h on the sea and constantly doing adjustments but getting the same results.



If anyone can throw some advise would be much appreciated



Thank you advance guys for your time to read this and eventually for any future involvement in solving this unpleasant situation



Respectfully



Manus
Not enough power mate !
I have a 2.6 rib tender fibre glass hull and tubes with a two stroke tahatsui 9.8 with a tiller extension on throttle so i can lay forward
And i have put a bit off wood on transom to higher prop as to low it does 30knts on my own will not plain with two adults
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Old 10 September 2019, 03:16   #8
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Originally Posted by carldalmas View Post
Not enough power mate !
I have a 2.6 rib tender fibre glass hull and tubes with a two stroke tahatsui 9.8 with a tiller extension on throttle so i can lay forward
And i have put a bit off wood on transom to higher prop as to low it does 30knts on my own will not plain with two adults
Engine wieghs 35kilos
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Old 10 September 2019, 03:22   #9
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OK so at 14st and no other kit... and knowing you could plane with a 3.5hp... plus you've tried altering weight distribution and know pressures are OK I can only think you need to try a lower pitch prop. The Mainer 3.5hp may have had a lower pitch prop enabling it to be in a better part of the power band at the point it was climbing onto the plane.

It would be interesting if you bought a 」10 rev counter/tach to know what revs you are getting to flat out but not on the plane.

With a 5hp supposedly modded to 6hp and a 2.85m Zodiac with two adults this planes well...



And a Tohatsu 5hp easily getting a 3.2m on the plane...

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Old 10 September 2019, 03:29   #10
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Have a look at the prop Manus and let us know the pitch or part number markings.
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Old 10 September 2019, 06:29   #11
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Is the default propeller that comes with the outboard.
7.8 diam and 8 pitch
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Old 10 September 2019, 07:04   #12
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I suspect you need a more powerful motor - you might get your 14 stones on the plane with a new prop, but once you add a picnic and some gear you may not be able to plane. i suspect the choice is 1. be happy and wet at displacement speeds or 2. buy a new outboard.
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Old 10 September 2019, 07:19   #13
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Hmm well gut feeling is the 8" may be too much and not allowing the OB to get into the powerband where it's able to push it over the hump onto the plane.

The standard pitch for many 15hp motors was always only one pitch up at 9" and they had triple the power to spin that pitch.

I've known folks with 6hp motors drop down to a 7" with a useful improvement in their ability to plane.
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Old 10 September 2019, 08:14   #14
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Hi Manus ..Fenlander is bang on the button as usual...with his suggestion to try a smaller prop

I did a recent test with a Honwave T38 airfloor as a couple of guys I know asked if it would plane with a 6HP. I said it would..most said..no chance !!!

I borrowed the boat and put my standard Yamaha 6HP four stroke on it and within a couple of meters she was happily on the plan. (I give the thumps up in the video when it goes on the plane.. around 10 meters after starting.)

It gave similar performance as my wee F Rib 2.75 with the outboard..example planed easily and went approx 15mph. I weigh 200lb ..so much same as yourself. The boat was empty except transom wheels..big bow dodger ..front seat and 10 litres of fuel.



The owner then tried and he struggled to get it on the plane. He weighs 50lb more than me. It did get on the plane after a long run but we could both see it was struggling and it was in fact labouring the outboard. I put a smaller prop on and he tried again. This time he got much the same performance as myself except he could only reach around 12 mph top speed.

For info..the prop I used for myself with the 6HP is the bog standard 7.1/4 x 8” pitch. It was running at 5150 revs to give me around 15 mph tops. (Max recommended is 5500 for my OB) The smaller pitch that gave my friend better performance was a 7.1/2 x 7” pitch and it too reached good revs with the smaller prop.

You are using an 8” pitch on a 5HP ..so you should get better performance with a 7” pitch but not as good a top speed. But as others have said.. it will be marginal performance and as soon as you get a few fish on board..it possibly wont plane the way it did on the way to get the fish.

Im pretty certain you will get it on the plane with trial and error and perhaps the smaller prop..but it could well be a bit disappointing with its performance.. however that is entirely up to you if all you want is to get it to plane with nothing else on board..
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Old 10 September 2019, 08:22   #15
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Guys

Thank you so much for your replies, I知 so grateful 😀

Everything you are saying make sense .........but still one aspect haunting me

How was possible to get on plane with my previous 2.3m inflatable using a 3.5hp mariner ????? And now I知 not capable 🙈
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Old 10 September 2019, 08:42   #16
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Hi again Manus Im afraid only you will know the answer to that one.

Here is some of my trials and results

FRib 2.75 empty boat will plane with a 3.3HP ..put anything else in boat example anchors and other essential stuff it wont plane.It easily planes with a 6HP and all my gear for a day

Seago slatfloor 2.7 planed with a borrowed 5HP but skittered around as it was a slatfloor so I didn’t do it often. It would not plane with a 3.3HP

Lodestar 3.2 airfloor easily planed with a 6HP would not plane with a 3.3HP
I have no experience of trying to plane a 2.5 with a 5HP or a 2.3 with a 3.5HP, so cant help you.


Totally irrelevant. But my 4.3m SIB easily planes myself and two other adults with its 25HP. With another adult it wont plane..yet I have heard some folk say they get them to plane with 7 folks on board.
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Old 10 September 2019, 09:40   #17
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>>>How was possible to get on plane with my previous 2.3m inflatable using a 3.5hp mariner ????? And now I’m not capable ��

First many thanks to The Gurnard for his real life experience demonstrating in marginal power situations prop pitch is important and just one inch can make a big difference.

Why was it OK with the 3.5 Mariner? Well I'm taking a guess it had a lower pitch prop so allowing it to get into its peak power range at the point it needed to overcome resistance and plane.

My gut feeling is the 5hp is being bogged down by that 8" just before it reaches the power band it needs to get onto the plane.

A 」10 tach may give a clue if you want to try that and report back the revs you are seeing... other than that it's taking a chance on a new prop around 」75 from Tohatsu.

If you do take the plunge with a new prop be cautioned only a genuine Tohatsu 7" can be assured to behave as 1" pitch lower than yours. Some aftermarket brands are inaccurately pitched or have a slightly different design that causes them to behave differently.

Of course in marginal power outboard situations nothing is guaranteed so we can only give best advice and hope it works out.
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Old 10 September 2019, 16:51   #18
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Just checking, what length outboard is the latest one, a short shaft as the name mentions doesn't sit as deep causing as much drag. The plate just above the prop should be close to level with the bottom of the hull. Next thing as a few have mentioned is prop, or should I say prop pitch. Reducing pitch will give a faster execration which sounds like what's needed to get this little boat out of the hole its digging as you open it full throttle. If like you say the bum is burying in and the bow rising, the engine should be trimmed in further.

On a larger boat a set of trim tabs would greatly help give lift at the back end. If it was me I would be looking around my garage for a couple of suitable L shaped brackets I could cut and fix to the stern to create the same effect of trim tabs, these wouldn't need to be very big at all to give extra lift on such a small vessel.
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