Repower with twin Yamaha high thrust 50Ts?

[Sea Pony]

The time has come to chuck my 23 year old Honda BF45s and I am considering replacing them with high thrust Yamaha 50Ts with 13-1/4" four bladed props. I would mount the new engines with shims to achieve more negative trim.

The Sea Pony (a 22' C-Dory) is used exclusively in the ocean and she lives in her saltwater slip. In addition to the engines, she has 4x18 gal gas tanks, 2 batteries and 2 kayaks (on an overhead rack) aft of the cabin. I have previously added 5 40# lead bars on the floor of the porta-potty space and gone to 4 bladed props to improve her handling going into the seas while avoiding hydrofoils and trim tabs.

My thinking is that the high thrust engines would allow me keep the bow as low as is necessary when going into the seas. As I have found no posts here by anyone who has gone to twin high-thrusts on a CD 22, I am hesitating to take the plunge. ..

I would be very grateful to hear any and all opinions, pro, con or indifferent, from the C-Brats.

[Wandering Sagebrush]

I have no opinion on the engine, but suggest you consider Permatrims and get rid of the 200lbs of lead. I'm happy with it on my Honda 90.

[Sunbeam]

Just some non-expert thoughts:

1) I don't totally understand about high thrust props on a planing boat. I thought they were more for things where you need more "torque," if you will. For example, a friend had one on a heavy (hull speed) sailboat, prior to installing an inboard, and it worked great for that.

Would 4-bladed props help?

2) On the Yamaha 50's: One thing to know (you may already have noticed this) is that with Yamaha, the 50 takes you up into a heavier block category than the Yamaha 40. With Honda, the 40 and 50 are on the same block, so there is no weight penalty for going to the 50 and it's 10 "free" horsepower, weight-wise. In contrast, the Yamaha the 50 is the smallest engine on the next block up, so you have the least horsepower but the most weight (of that block category). With more weight on the stern you might start to get into a never-ending cycle (more stern weight, needs more power/lift, etc.)

[Aurelia]

If I wanted more stern lift to push the bow down I would start with a permatrim on the motor or motors regardless of motor or prop. I would then tackle lightening the stern or moving existing weight forward or adding trim tabs and treat those two tasks about even in potential effectiveness. I put my house batteries under the berth adding 145lbs up there and still need the permatrim and trim tabs on the 19.

We run a 4 bladed prop for added stern lift as well and it provides a bit more help (I think) and we run a low pitch as well for heavier loading overall. That Yamaha 50 is the same block as the 70 so are are getting the most weight for least power for the brand. How does it compare to the weight of your current motors?

The high thrust units add further weight to your stern but would likely provide some nice controllable lower speed thrust. So you want the high thrust only for added stern lift?

Greg

[Sea Pony]

@ Wandering Sagebrush

I love my C-Dory but for the 20 years I have owned her I have tried to improve her handling going into lumpy seas. One problem was that her light bow was being thrown up exposing her flat bottom to the wave as well as sideways throwing me unpredictably off course and even dangerously parallel to the oncoming waves. The worst situation is when waves are coming from more than one direction which is not uncommon off the Oregon and Washington coasts. The lead ballast up front helps a great deal by adding weight and inertia to the bow and I shall not be removing it regardless of whether I decide to re-power with thigh-thrust engines or not. I did try hydrofoils on her but soon removed them because they seemed to make her handling sluggish in the conditions I am concerned about.

Thanks for your reply.


David

[Sea Pony]

@ Sunbeam

Thank you for your reply. High-thrust engines should provide upward instantaneous upward thrust and should get you on plane quicker and at a lower or zero speed. This is in contrast to trim tabs or hydrofoils where the upward thrust starts at zero and only increases with the speed.

Perhaps the main advantage I would see in going to high-thrust engines is the larger diameter props they can handle. Negative trim provides an upward thrust which directly raises the transom. In addition to this it is my understanding that even at zero (or level) trim turning propellers provide upward thrust. This apparently has to do with the fact that on the downward spinning side of the propeller water is being pushed against water below while on the upward spinning side water is being pushed against air (This explanation is probably too simple. .. Any physicists Here?). In both cases (at least at lower speeds) the upward thrust depends on the surface area of the prop. A 4 bladed prop will generally have a larger surface than a 3-blade. And a larger diameter prop will also have a larger surface area than a smaller one. This at least is the way I am thinking. ..

Your point about the extra weight of the Yamahas is of concern to me. I would like to go with Yamaha because I believe the fresh water flush system and corrosion protection are better than Honda. I would expect that the extra weight of the Yamahas would more than balanced by the 50T's extra thrust. .. If not, I may be out of luck as my practical choice here in Newport OR is limited to Honda or Yamaha.

Cheers, David

[Will-C]

Do you know anyone who actually uses these high thrust engines on a boat similar to yours? It was my understanding these engines were designed for work boats or larger heavy pontoon boats etc. I would opt for regular 50 hp units or even 40hp units as they would save 50 plus pounds of weight on the transom. I would also spring for a couple of Permatrims as there are documented cases of handling improvements and providing lift to the stern. I would talk to Andrew Munao jr. at Sim's Yamaha about the choice of motors for your boat. 1-800-213 3323 ext 10 or email him at andyjr@shipyardisland.com These folks seem to be straight shooters and know Yamaha's pretty well. Buying two engines that may or not be the best of choices could be an expensive mistake. It also might make the boat harder to sell in the future should change suddenly interrupt your life.. Just JMHO here.
D.D.

[Wefings]

High Thrust motors have a bigger diameter gearcase and likely will be of no advantage on a light weight C Dory . Regular ratio/gearcase motors with carefully selected props would be what I would opt for . Call Powertech [ ptprop.com ]in Shreveport La. for great selection of props and lots of knowledge . Transom wedges are a great [and simple] addition for pushing the bow down. 4 blade props for low speed handling are the way to go , as they have great "bite" and good midrange performance which is what you want for ocean use. I would start there .
Marc

[Sea Pony]

@ Wefings

Transom wedges and 4-blade props are a given, Marc. But, assuming that everything else is equal, would not hi-thrust engines with lower gearing and larger diameter propellers magnify the effect of these two measures? At least in the 0-20 mph speed range that is of concern to me? -David

[Wefings]

I don't see an advantage on the twin engine boat .There is no shortage of thrust on a standard twin setup on a C Dory.There is no way of knowing quantitatively as you would have to do a apples for apples comparison which aint gonna happen. You will however get lots of speculation and expert opinions ...................
Having set up a number of them, I don't see a better mousetrap with high thrusts .
Marc

[Sea Pony]

@ Wefings

Thanks for your replies, Marc. I just found this head to head comparison test between a Honda BF60 (Standard) and a BFP60 (High Thrust):

http://www.tradeboats.com.au/news-reviews/2013/6/honda-bfp60-power-thrust-outboard-engine-review/

The lower planing speed and low-end control sound very promising for my needs. Honda does not make a high thrust 50 so I guess this is as close to apples to apples as we can get. .. Too bad the test wasn't on a C-Dory. But I don't reckon there are many in Australia!

This thread from another forum is also interesting:

http://continuouswave.com/ubb/Forum4/HTML/003071.html

Since I am thinking to buy either Yamaha 50s or 50Ts and I would like more transom lift than I have now with my 4 bladed BF45s, do you see any reason not to go with the 50Ts and hope they help? The price difference is not much. (It would be more economical, of course, just to buy a pair of regular BF 50s and keep my existing instruments and controls. The extra horsepower might help some. ..) -David

[rogerbum]

I don't understand why you don't want to use trim tabs and/or hydrofoils (like the Permatrims). With ths cost of lead these days, you could probably sell the 200# of lead you have in the boat and buy the permatrims.

[Sea Pony]

@ rogerbum

In my opinion nothing will really do much to stop a lumpy sea from knocking the featherweight bow of a C-Dory every which way except mass (to provide inertial resistance to a change of direction) up front. Permatrims apply a downward force on the bow from the transom where they don't have much leverage. They will lower the bow and give it a better angle of attack but it is still a very light bow hitting a very heavy wave. And they won't do much when a wave strikes the bow laterally which is usually what happens since it is not a good idea to attack a wave head on. Since adding the lead ballast bars, wave splashes often totally soak the boat when she carves the waves but she stays on course and passengers are knocked around a lot less. My lead is worth its weight in gold thank you.

[Aurelia]

If you are battling these conditions at less than 10 knots then I see your point on the weight in front. Just would be nice if it were multipurpose.

We ran about 45 miles recently with 65 additional gallons of fuel and water in portable jugs on the floor of the boat moved forward as much a possible in the cabin. We were ripping along at 13 knots and came upon the first sizable boat wake coming toward the bow at an angle. I was not making any moves and Cindie said aren't you going to do something about that wake? I said "we already did", having felt the weight of our added ballast already in how the boat was performing (slowly).

Instead of bouncing up and off the wake as we normally would, Ari just tractored right through it like a small ship of sorts and Cindie was very surprised. Amazing what some added weight can do.

We were carrying most of those jugs for my sisters boat and they later said they felt bad for us carrying the load and it must have made our boat unstable as it does theirs when loaded heavy. I told them quite the contrary, especially with the load largely below the water line. It did make us slower however taking our cruising speed down from 15 knots to 12-13 knots.

We love our C-Dory,

Greg

[rogerbum]