Change or not to change outboard propeller?

[Mulkari]

I have a 9.5 m long relatively lightweight sailing catamaran around 1500 kg displacement at average load. It is powered by 6 hp Mercury long shaft outboard motor located in the middle in retractable sled. In flat water no wind conditions it goes around 6.5 - 7 knots at full throttle. Problem is if there is some strong headwind in waterway too narrow to tack upwind speed drops to around 3 knots or in winds around 30 knots it becomes almost impossible to make any progress at all and motor sounds like it is struggling and can't reach nowhere near max rpm.



Motor has stock propeller it came from the store. As I understand these stock propellers are meant for small planing boats going 12 - 15 knots not for relatively heavy displacent boat with large surface area for wind to grab. I wonder would it be worth to change propeller to one with lower pitch? Would it make noticeable improvement or my motor is just too weak and I have to live with occasional inconvenience?



I know there are Yamaha 9.9 high thrust outboard, but it is expensive and damn heavy at around 50 kg while my Mercury weighs only 30 kg. I really like the portability of it. I can easily lift it off the mount and lock in a cabin. Leaving motor on it's mount visible to anyone just asks for it to be stolen when boat is left unattended. I'd also like to keep unnecessary weight to a minimum on a light catamaran.

[Benz]

My understanding is that a motor must be geared differently for a high-thrust prop, but I'd ask a Merc dealer.
If you cannot make way to weather in 30 kts, you're in the same situation as most of us. I stay put when it blows that hard.

[Chotu]

Allow me to introduce you to the Bible. This is a thread I started on the same topic. Very informative.


https://www.cruisersforum.com/forums...to-232192.html

The cliff notes summary is you need a lot more surface area on the prop. It’s not just about the pitch.

[Mulkari]

Originally Posted by Chotu

Allow me to introduce you to the Bible. This is a thread I started on the same topic. Very informative.


https://www.cruisersforum.com/forums...to-232192.html

The cliff notes summary is you need a lot more surface area on the prop. It’s not just about the pitch.

I looked it up and tons of useful info there, thanks. Size of the prop I could fit is limited by anti ventilation plate on the outboard leg, original 3 blade prop is already near the limit. I'm thinking a 4 blade propeller with shallower pitch might be worth a try. It is only thing that could be done while keeping the same motor. I would get a more blade area and in high load situations like going into strong wind motor would reach higher rpm end produce more power and hopefully also a bit more thrust.

My understanding is that a motor must be geared differently for a high-thrust prop, but I'd ask a Merc dealer.

I have asked a dealer from who I bought my outboard, he told that most likely I would get more fuel consumption and slower top speed. However dealer is mostly small planing power boat guy so he may not be that knowledgeable about relatively heavy displacement sailing boats.
I know a proper big high thrust prop require lower gear ratio however such high thrust gearbox is not available for 6 hp Mercury motor.

If you cannot make way to weather in 30 kts, you're in the same situation as most of us. I stay put when it blows that hard.

Under sail it is no problem to tack and make way into 30 knot headwind if there is enough room. Problem is if there is very narrow channel like river mouth or small harbor entrance where even motor assisted sailing and tacking is not an option.

It is not really a huge issue, in 4 years I have this catamaran I have only once or twice encountered situation where I had to anchor and wait few hours for wind to calm down. If there is no easy way to improve thrust of my motor then I can live with that. Bigger issue is prop ventilation when it pops out of water when exiting from harbor into sea with short steep waves, but nothing much can be done there, different prop or more horsepower will not help there. It is a price I have pay for having a lightweight catamaran with good sailing capabilities.

[Cadence]

Originally Posted by Mulkari

I have a 9.5 m long relatively lightweight sailing catamaran around 1500 kg displacement at average load. It is powered by 6 hp Mercury long shaft outboard motor located in the middle in retractable sled. In flat water no wind conditions it goes around 6.5 - 7 knots at full throttle. Problem is if there is some strong headwind in waterway too narrow to tack upwind speed drops to around 3 knots or in winds around 30 knots it becomes almost impossible to make any progress at all and motor sounds like it is struggling and can't reach nowhere near max rpm.



Motor has stock propeller it came from the store. As I understand these stock propellers are meant for small planing boats going 12 - 15 knots not for relatively heavy displacent boat with large surface area for wind to grab. I wonder would it be worth to change propeller to one with lower pitch? Would it make noticeable improvement or my motor is just too weak and I have to live with occasional inconvenience?



I know there are Yamaha 9.9 high thrust outboard, but it is expensive and damn heavy at around 50 kg while my Mercury weighs only 30 kg. I really like the portability of it. I can easily lift it off the mount and lock in a cabin. Leaving motor on it's mount visible to anyone just asks for it to be stolen when boat is left unattended. I'd also like to keep unnecessary weight to a minimum on a light catamaran.

JMHO, Live with it, your under powered.

[Boatguy30]

The prop used on the Tohatsu sailpro is the one you should try first. It pushed my 34' cat at 5.2 knots in flat water. I also have a Yamaha T8. The older T8 is same motor and the new 9.9 just a different carb.

The Honda 8 or 9.9 is actually, overall a better motor and has a much higher and regulated battery charging output.

[barnakiel]

The prop on Tohatsu sail model is good but it is a wee engine otherwise.


A 8 hp yama could fit your use and last well too. See of a Tohatsu prop can be fit on Yama 8hp.


b.

[waterman46]

Originally Posted by Cadence

JMHO, Live with it, your under powered.

Exactly, you beat me to it. 6hp is way too small for your boat. OK for in and out of slip in normal conditions but will never be able to push a big windage boat upwind in 30 knots. But you don't have to live with it! 15hp would be proper, and would make a huge difference.

Having to motor upwind in 30 knots is rare, but I have kept my boat in the past at one location which has a regular occurrence of this. Oyster Cove Marina / Brisbane Marina, SF Bay. Narrow channel maybe 500 yards, to return to slip you must motor dead upwind and wind regularly averages 20 to 25 in summer late afternoons, sometimes gusts to 30, just when you want to return to slip. No problem for 60hp monohull with probably same windage as your cat, but 6hp is a non-starter.

[tomfl]

There is a reason the 9.9 Yamaha is the gold standard of outboards to power boats like yours. Not just the long shaft and the high thrust (read low gearing); but most important the ability to put an elephant ear prop on them. My elephant ear props are ten inches in diameter. Bottom line is size matters. Think there is a 7.8 inch prop that may fit your 6hp Merc.

[Mulkari]

Originally Posted by waterman46

Exactly, you beat me to it. 6hp is way too small for your boat. OK for in and out of slip in normal conditions but will never be able to push a big windage boat upwind in 30 knots. But you don't have to live with it! 15hp would be proper, and would make a huge difference.

Yeah, my motor is underpowered, but I mostly use it only to get in and out of my harbour often just 100 meter or so to get out of wind shadow produced by trees. If wind dies and I have to get back to harbour it easily cruises at 5.5 knots in calm water, max speed 6.5 - 7 knots. Good enough for that. It was perfectly adequate for my Gotta canal trip in Sweden last summer. My cat is fairly minimal boat oversize beachcat similar to Stiletto 30 and the like. Only valid use case for more powerful motor would be very narrow channel like less than 50 meter, too narrow to tack upwind with strong headwind and fairly calm water. All other times a bigger motor would just be more weight, require more fuel again more weight.

Beating into steep waves would not be possible anyway because too much prop ventilation even with long shaft. More horsepower would just produce more aerated water and that's it. That's why I think about different prop, it would not add any extra weight and be fairly inexpensive upgrade. If I could get 20% - 30% more thrust with another prop it would be perfectly fine.

[Mulkari]

Originally Posted by tomfl

There is a reason the 9.9 Yamaha is the gold standard of outboards to power boats like yours. Not just the long shaft and the high thrust (read low gearing); but most important the ability to put an elephant ear prop on them. My elephant ear props are ten inches in diameter. Bottom line is size matters. Think there is a 7.8 inch prop that may fit your 6hp Merc.

Too bad there isn't a low gear ratio gearbox available for my 6 hp Mercury. Yamaha 9.9 high thrust would be perfect if only it would weigh less.

[a64pilot]

Get the lowest pitch largest diameter four blade prop that will fit.
BTW, my limit with my 40’ mono with a 44 HP inboard Diesel is 25 kts, at that point I can maybe make 3 kts, so your not bad off, not at all.

[Bunji]

Catamaran design is of course a compromise, placing the motor in a pod (nacelle) in the middle of the bridgedeck isn’t ideal for obvious reasons and designers know this, from a performance sailing perspective it maybe the best option given that the motor is primarily used to enter/exit marinas, so if u were to mount the engine on the back of either hull (as many are) how much do u think that will effect sailing performance versus the much better drive, next to no cavitation, ventilation

[Mulkari]

Originally Posted by Bunji

Catamaran design is of course a compromise, placing the motor in a pod (nacelle) in the middle of the bridgedeck isn’t ideal for obvious reasons and designers know this, from a performance sailing perspective it maybe the best option given that the motor is primarily used to enter/exit marinas, so if u were to mount the engine on the back of either hull (as many are) how much do u think that will effect sailing performance versus the much better drive, next to no cavitation, ventilation

I have stern hung kickup rudders, taking up transom space. Even if I could put motor there not sure it would be better. I have sailed on monohull sailboats with transom hung outboard and it also ventilated badly in waves. Whenever boat crossed wave the bow would drop and outboard leg briefly pop out of water. My current setup has motor placed about 1.5 meter inside from sterns. If anything it may be better position than on the stern since weight is more central and motor is closer to a center of motion. I put up a drone picture of my boat so it is easier to visualize how everything is set up.

My workaround to to prop ventilation if I have to beat out of harbour or across the sandbar in headwind and steep waves is to motorsail. Then sails produce most of the drive to punch through the waves and motor is at half throttle to help tacking and when it ventilates I still have drive from the sails.








On a larger cat ideal setup for outboards would be inside the hull in a wells where saildrive normally would go. All the benefits of outboards without the disadvantages of heavy and expensive diesels and drag producing drive legs.

[Bunji]

Ok, looks like a crowther (shockwave) or similar, not a lot u can do, as u know it’s primarily designed to sail (fast) motor placement is somewhat and after thought ((compromise) I have a similar design (bob oram) solid bridgedeck more accomodation which I’m converting into a powercat, added 1.5 metres aft and in doing so took most of the rocker out of the ass end (more buoyancy) 2 x 20 hp xtra long legs (27 inch)