More Power

[shaunh]

Recently went out through Cape Cod Canal to Cape Cod Bay along the Plymouth coast into very strong Northeast head winds. Seas were running 6 to 8 feet. Everything was just too choppy to stay out long and turned back after the tide changed so we could take advantage of the canal current running east to west. Estimate current at 6 or 7 knots minimally.
Missed the turn into Onset Bay where my mooring ball is located and had to reverse direction and battle the currents to get back to the bay entrance.

Could not overcome the combination of current and the northeast headwind and literally were going backwards or at best standing still at full throttle. Spent an hour trying and eventually needed a tow from a passing power boat.

This is a 1985 Catalina Tall Rig with a 21 hp Uiversal M-25 engine and a 2 blade prop.

Is there anything I can do to gain more power for crucial times? Plannng to change prop out to a 3 blade desiung but doubt it will do much. What are the critical specs needed to choose the right prop?

 

[Bob V]

If the current was actually 6 - 7 knots

you were not taking advantage of the current you were placing yourself at it's mercy. You should not be in a current that fast in close quarters with a 30 sailboat. Timing is everything. I can't imagine how one could come into a slip at those current speeds no matter what direction it was flowing.

Changing the prop or anything else you might do to increase power is only going to have a small effect on your top speed. You're never going to get that boat up on step. Even if you had a fast boat like a 30 ft Bayliner it would be reckless to dock in such fast currents.

 

[shaunh]

Re: If the current was actually 6 - 7 knots

Bob,
Thanks for the reply. I was not clear about the situation. My appologies. I was not trying to come into a dock or a slip. The turn I was refering to was the entrance to Onset Bay where I have a Mooring Ball set up with the town. It is a well protected harbor and the surrent conditions are completely different than in the channel.

Sorry for the confusion but again thanks for replying

 

[Bob V]

Actually, your first post was pretty clear.

I was reading too fast for my comprehension.

Still, I would tend to stay out of 6-7 knot currents with a boat that can probably only make 6-7 knots speed. It didn't sound like you had many options on that particular day. Glad to hear that your mooring buoy isn't in that strong current.

Good luck.

 

[Jim]

I am surprised you couldn't make way against the current. I would think you could move at least one know. I have made way against the tide at Hell Gate before.

I would change to a 3 blade. I think you'll be suprised.

 

[Alchemist]

Jim's right.

You will be surprised when you change to a three bladed prop. Your boat will sail like crap, and won't motor any faster in flat water.

Independent of how many blades your prop has, hull speed is hull speed is hull speed, and the correct two blade prop will get you up to hull speed with your M25XP engine. The primary advantage a three blade prop will give you is in acceleration, not top end. Three (or more) blades only give you an advantage over two blades when motoring into chop or if you have to do an emergency stop.

Because a three blade prop has more surface area than most two blade props, it drastically increases drag (especially noticible when sailing down wind), so you sail slower under most wind conditions (especially in winds under 12 - 15 knots) with a three blade sea anchor uinder your boat.

A three blade prop will not give you any advantage motoring into current as current can be thought of as a moving carpet, and your boat speed through the water is limited to hull speed while your speed over the ground is limited to hull speed minus the curent (using vector math).

That having been said, having the correct diameter and pitch is critical for whatever prop you have under your boat, and you should be able to achieve hull speed (6.7 knots) at 75 - 80% of WOT (Wide Open Throttle), or about 2400 - 2600 RPM. If you can't achieve that speed under those conditions, you have the wrong prop.

Steve
Alchemist C-30T #4764 ('87 Mark II)
Alchemist C-320 #909
Alchemist C-380 #71

 

[Dan from Nahant]

Buy an Eldridges

The CC canal runs a current close to many boats hull speed. A bigger prop or engine will not defeat the laws of Physics which determine hull speed. At the beginning of every season I put a new Eldridge in the Nav Station. It will tell current speed and direction of the CC Canal for every hour of the entire year.