Slow speed on a cat under power into headwind

[CAPTAIN FANTASTIC]

Don't you mean fine pitch propeller? olewill (often wrong)

I was thinking that the current configuration of the propeller may be too fine and therefore "slip" in water; a coarser prop will "bite" more and will reduce the engine rpm, converting into forward thrust. It will be good to check the pitch of the current propeller and then take a view.

 

[Boathook]

I was thinking that the current configuration of the propeller may be too fine and therefore "slip" in water; a coarser prop will "bite" more and will reduce the engine rpm, converting into forward thrust. It will be good to check the pitch of the current propeller and then take a view.

Ideally you should have fine pitch props on standard outboard engines when being used on a displacement boat. Coarse props may give good initial bite and a high speed in flat seas with no wind but in anything else they won't develop the power. I would expect the 26ft Prout Sirocco to have a maximum speed of 6 to 7 knots under power and this should be achieved at full engine rpm. My Cat used to have a Yamaha 30 2 stroke with a dual thrust prop (fine pitch) which gave a top speed of around 6.5 knots. Against the wind the speed does drop of especially with wind against tide that you had.

 

[cueball]

I'm thinking replace the twin 2 stroke 15hp mercury with twin extra long shaft 9.9 Yamaha high thrust, I have found two 2010's at a reasonable price. Now does one need to be counter rotating or can I get away with it? They are situated about 14 inches apart.

 

[William_H]

I'm thinking replace the twin 2 stroke 15hp mercury with twin extra long shaft 9.9 Yamaha high thrust, I have found two 2010's at a reasonable price. Now does one need to be counter rotating or can I get away with it? They are situated about 14 inches apart.

I don't think counter rotating is necessary. Probably not available anyway. olewill

 

[Neeves]

Ours, the props, are counter rotating, though whether it is critical - I don't know. Ours must be about 4m apart. It does have its amusing side effects as if you mix the props up (when you take them off to service the anodes) your throttles work in the opposite manner to normal. Don't ask how we know.

Edit - that's wrong - the props go the same way round the blades are reversed.

You only make that mistake once.

Jonathan

 

[alahol2]

I'm thinking replace the twin 2 stroke 15hp mercury with twin extra long shaft 9.9 Yamaha high thrust, I have found two 2010's at a reasonable price. Now does one need to be counter rotating or can I get away with it? They are situated about 14 inches apart.

You may well find you can get away with just the one 9.9 Yam. Like NealB we had one on a 32ft Iroquois and never felt the need for more power. Incredible outboards.

 

[wotayottie]

Hi All,
I have recently purchased a small 26ft prout sirocco. Great boat and very convenient for the east coast.
She has 2 1994 mercury 15hp 2 stroke outboards, the problem I have is when we have to motor into strong headwind she is painfully slow, any suggestions? Is it time to repower? Should I look at more hp? I thought two 15s should have been plenty, maybe il start with props? Or is this a characteristic of cats? Thanks in anticipation

Yes it is characteristic. I has a prout quest with a 19 bhp diesel and I could not motor into 20kn or more. Beating into any sort of chop it was a motorsailer.

 

[alahol2]

Yes it is characteristic.

It may be a characteristic of Prouts, it certainly isn't one of cats in general.

 

[cueball]

I really like the idea of 2 outboards, one for safety i.e. if one fails I have auxiliary and two, so much easier maneuvring!

 

[Trident]

At 14 inches apart you won't get great manoeuvring - they're too close to get the one side moving forward and one back for the typical twin engine turn on the spot that an engine in each hull will give. It's your boat but I would try one on the nacelle in the middle - high thrust, extra long shaft and see how you find it - you can always buy another if you feel you need more

 

[Muddy32]

My Comanche has 2 12.5hp diesels, one in each hull. one engine will push me at 4.5 kts normally at cruising revs.
Both engines into 20+ kts make 4-5kts but that as a VMG is very respectable. Tide makes difference with SOG but that will always be the case. Sea conditions make a difference to bounciness and passenger comfort= complains!!

 

[Puggy]

Before spending a lot of money on new outboards, I do advise you to try some new props first (or at least do some research into what pitch/diameter you have at the moment.).

If they are the standard props that came with the engines when new, they are almost certainly designed for a planing craft - with the prop moving through the water at 15 knots+ at full revs (when your two stroke is producing full power). If they are the standard props, they will be too coarse (over-pitched) and will not allow the engine to get into the power band and produce full power. This will become particularly apparent when the boat is slowed (eg when trying to motor into a head sea/head wind), and the engine can fall behind the power curve and lose even more grunt,

All outboard manufacturers produce high-thrust type props for this application - often they have large more symmetrical blades and a special faring to help clear the exhaust and prevent prop cavitation at the lower speeds they operate at.

If this doesn't work, look at the depth of the engines in the water - as other posters have said if the engines are too high, they will cavitate and lose grip as the boat pitches. The solution is either mount them lower (use a sort of reverse jacking plate that you can lift when the engines are not required) or swap for long shaft or ultra long shaft versions.

Good luck.
Puggy

 

[cueball]

I will try props first. Not at the boat till the weekend. Thanks all for the great advise

 

[thinwater]

For over ten years, I had a 32 foot Comanche cat, fitted with a Yamaha 9.9 hp high thrust engine.

These engines have a high reduction, so drive a nice big, slow turning prop, and drove that boat unbelievably well.

I'm sure it's your fast spinning, little whisk, that is the problem, not lack of HP.

^^ This. The Yamaha 9.9 is popular with many cats up to 36' because it has much better low speed grunt. They would push my PDQ 32 into anything as fast as I would want to hit the waves (7 knots). I motored into 30 knots many times.

 

[ex-Gladys]

Mate of mine has Martyn Smith's 42' home built cat on loan, the rig on that is huge, and the single 9.9 Yam 4stroke copes manfully in anything up to about 15 kts on the nose... It is very light though (about 2.5 tonnes)

 

[cueball]

Wow 42 ft with a single Yamaha 9.9! If I changed from 2 x 15hp 2 stroke mercury to one 4 stroke Yamaha 9.9hp I would also save weight. In my book that means sailing faster ��

 

[Neeves]

Wow 42 ft with a single Yamaha 9.9! If I changed from 2 x 15hp 2 stroke mercury to one 4 stroke Yamaha 9.9hp I would also save weight. In my book that means sailing faster ��

I don't think you would notice the difference.

If you want to save weight, go for an alloy anchor, replace your spare anchor with a different style of alloy (that's an alloy Spade and a Fortress). Reduce your chain size - use G70 - that will probably save you 1kg/m (we replaced our 8mm chain with 6mm chain, we use all alloy anchors). Buy a de-sal unit - carry water, you carry weight, save water, shower with a friend. Cut back on wine, learn to love spirits. Leave the outboard for the tender at home, cut back on gym fees - learn to row . Cut back on books - use Kindle. Cut all toothbrushes in half (its purely psychological). Look at your tool box, how many 10mm spanners do you have, how many do you need etc.

Jonathan

 

[cueball]

An update for you all.
I have taken all the excellent advise on-board and purchased a 2011 Yamaha 9.9 high thrust 4 stroke extra long shaft to replace the 2 x 2 stokes which I will sell. I will post with results once received and fitted.
thanks again.

 

[geem]

Wow 42 ft with a single Yamaha 9.9! If I changed from 2 x 15hp 2 stroke mercury to one 4 stroke Yamaha 9.9hp I would also save weight. In my book that means sailing faster ��

On a small cat weight is very important. I would not have two outboards on a 26 ft cat. One 9.9hp high thrust will be fine. A reefed main and outboard should push you to windward well enough

 

[cueball]

On a small cat weight is very important. I would not have two outboards on a 26 ft cat. One 9.9hp high thrust will be fine. A reefed main and outboard should push you to windward well enough

From research it seems that many people have just a single Yamaha 9.9 high thrust with Cats bigger and heavier than mine.......must admit still a little anxious as I'm reducing my current 30hp set up substantially.