How fast?

lightning · 25 July 2009, 23:40

......should my Zodiac 2.85m SIB with Mercury 6hp engine go?
I was on the canal today and the boat does about 2mph on tickover. Increasing the revs a bit produces 4mph or so with no effort and little wash.
However increasing the RPM beyond this has little effect. A large foaming wave builds up behind the boat, the stern lowers and the bow rises 6 inches or so. Further RPM (say up to 70% full throttle) just sinks the stern lower, raises the bow more, and produces a large following wave and wash. No real speed increase. Maybe another 1 or 2 mph.
I only tried this briefly on a section of the canal with steel banking.
However, after being waved past a couple of slow moving canal barges I was really struggling to make any real speed over them to get past.
What could be wrong? I was sat at the stern and my wife on the seat near the front. I tried altering the angle of the engine but this made no noticeable difference.
The cavitation plate is a bit lower than the recommended range of 1.5-2 inches below the bottom of the transom, but it's only just under 3 inches (less than one inch too low?) Should I try a spacer to raise the engine a bit or is there nothing wrong here?

Chris · 26 July 2009, 07:35

You are bascially reaching the limit of the hulls displacement speed and as such it is trying to get "over the hump" and onto the plane I would suggest you may need to move some weight forward to try and achieve this. However it may simply be that your Sib is not going to plane with 2 adults on board with a 6hp pusing it along. You may need a larger engine.

chewy · 26 July 2009, 07:39

Doel fins might help? Cheaper than a new outboard.

Poly · 26 July 2009, 08:26

Have you "played" around with the trim settings?
Its not stuck on "shallow drive" is it?

That said - I think 6HP is going to struggle to plane.

lightning · 26 July 2009, 12:16

I've seen some videos on YouTube (type in Mercury 6hp or Honda 5hp etc) and there are videos of small SIBs with these engines, granted you are not going to be doing 20mph but they seem to manage 10mph with no problems. Many seem to have 2 people on board.
Clearly my boat is OK, I mean there can't be anything "wrong" with it, but I was hoping it might manage more than 5mph. I may as well have gone for the lighter Mercury 3.5hp if the upper 50% of the engine RPM on my 6hp don't really do anything!

codprawn · 26 July 2009, 12:20

You need to get someone to hang right over the bow as far forward as they can - as it starts to plane they need to move aft. That's assuming it will of course!!!

And don't think you may as well have a smaller engine - if you are at sea and there is a fast tide running yopu will need every bit of extra power..........

lightning · 26 July 2009, 12:34

So if I understand it correctly, the boat will achieve its "displacement speed" of say 5mph with little power, maybe even a 2.5hp. To achieve more you have to get it to "plane" which guess means the hull lifts out of the water and it rides on the V-keel? And then it might do 8-10mph, but you may need a lot more engine power for this.
The maximum recommended engine power for this boat is 6hp, with 8hp as a "maximum safe limit" according to the instructions.
I had no idea there was this "transition" where the boat goes "on the plane". It explains a lot. Over the years I've been on a few small boats (engine under 5hp) including my dad's SIB years ago, and on all of them, increasing the engine RPM past about 50% just makes more noise with little extra speed.

PeterM · 26 July 2009, 12:38

you need to get the hull to the stage where it is producing 'lift' out of the water , but with lift you have drag - so getting over this 'hump' is the hard bit.

I'd think with a bit of moving weight around and playing with the trim you shoudl get on the plane with 6hp ( unless the floor is sucking itself to the water).

lightning · 26 July 2009, 12:49

What do you mean by "playing around with the trim"? Do you mean the engine angle/height? I can't think of anything else that I could alter.

chewy · 26 July 2009, 12:52

The engine angle.

lightning · 26 July 2009, 13:01

Oh, OK. Forward (so the prop is nearer to the boat) or back?

astra · 26 July 2009, 13:12

Hello all, i have a seago 2.7 airdeck with a 6hp tohatsu four stroke and i belive it can plane it just takes a bit of shifting around and is a lot better when using a tiller extension,the video is the first day we took it to bristol docks and is a lot better now we have played with adjustment on the engine. hope the link works,well i think it planes guess someone will say if it dont?

http://www.youtube.com/watch?v=MwCe8ivXm-M

astra · 26 July 2009, 13:17

Oh and top speed on gps was 13mph with me and the mrs on.

codprawn · 26 July 2009, 13:23

Quote:

Originally Posted by astra

Hello all, i have a seago 2.7 airdeck with a 6hp tohatsu four stroke and i belive it can plane it just takes a bit of shifting around and is a lot better when using a tiller extension,the video is the first day we took it to bristol docks and is a lot better now we have played with adjustment on the engine. hope the link works,well i think it planes guess someone will say if it dont?

http://www.youtube.com/watch?v=MwCe8ivXm-M

See it's people like you that give us ribbers a bad name - that is lunatic behaviour. Look at your wake - it could easily have sunk that ex RN vessel. It's because of people like you that they want a speed limit on the Thames..........

And yes you are on the plane - slightly bow high though!!!

Poly · 26 July 2009, 16:38

Quote:

Originally Posted by lightning

Oh, OK. Forward (so the prop is nearer to the boat) or back?

You will find people generally refer to trimming up (prop nearer water) or down. If you search here you will find plenty about trimming up/down depending on conditions and to help get over the "hump" etc. Generally to help get a boat on the plane you trim down (leg closer to transom).

However once on the plane, and to get best speed / comfort in waves etc - people normally want the engine trimmed up a bit more. On a big engine this can be adjusted on the "fly" with power trim and tilt, with small engines with manually adjusted trim you have to set the trim for a trip (or leg of a trip) and accept the compromise you get. Have you found the pin that lets you adjust the trim angle?

I would also raise the engine and inch or so - as if you are struggling for speed extra drag isn't going to help. Although I doubt that this alone will be enough.

m chappelow · 26 July 2009, 21:58

mate of mine can get 14 mph out of a 3.metre zodiac and a 4 hp mariner with one person ,i use to have a zodiac 12foot sib and i could only just get it to plane with one up and a johnson 4 hp ,a big difference with the engines and props ,,,also you say you were trying to pass some other boats ,you can get some weird water flow effects in narrow canals a barge or narrow boat with a big prop will suck water from in front of the boat even up to 10 meters in front and its not uncommon for a smaller boat in front to run out of water and ground out for a few seconds if near the sides or bank its called the canal effect and causes large ships problems. in effect they suck that much water,past the prop that the boat will drop and ground ,,,it can have that much effect that a boat will steer its self without touching the banks ,so that wont help if passing ,,,,in fresh water the prop slips a bit more too than in salt water ,,as its less dense.

thornbackflound · 27 July 2009, 11:06

Hi all any idea what top sort of speed i should get with my SEAGO ,FLAT BOTTOMED 2.7M SIB AND MARINAR 3.3 HP,?

m chappelow · 27 July 2009, 17:11

Quote:

Originally Posted by thornbackflound

Hi all any idea what top sort of speed i should get with my SEAGO ,FLAT BOTTOMED 2.7M SIB AND MARINAR 3.3 HP,?

bit hard to say but i think you would be struggling to get on the plane with an engine that small ,but if as you said a flat bottomed boat you might just stand a chance ,

thornbackflound · 27 July 2009, 17:31

Quote:

Originally Posted by m chappelow

bit hard to say but i think you would be struggling to get on the plane with an engine that small ,but if as you said a flat bottomed boat you might just stand a chance ,

Arh yes it be a slow ole engine ,good work horse tho ,pretty reliable and thats what/s needed ,just wondered how far off a 3.3 is to a 4hp

lightning · 27 July 2009, 19:55

About 0.7hp would be my estimate.