Gear Ratio
- Thread starter waterking2
- Start date
If the boat is on the hard, you can rotate the engine with someone looking at the prop.
Rotate the shaft so that one blade is vertical, mark that blade. Mark the top of the shaft near the transmission. Mark the top of the flywheel.
With the engine in gear, rotate the flywheel one revolution, where is the mark on the shaft? It may be easier to see the mark on the prop.
Rotate the flywheel another revolution. Where are the marks? If the marks on the shaft and prop have returned to their original position, then the transmission is 2:1.
If the prop and mark are not back at the top, rotate until the marks are again a the top and note the mark on the fly wheel. That will give you the ratio. Repeat for reverse.
Rotate the shaft so that one blade is vertical, mark that blade. Mark the top of the shaft near the transmission. Mark the top of the flywheel.
With the engine in gear, rotate the flywheel one revolution, where is the mark on the shaft? It may be easier to see the mark on the prop.
Rotate the flywheel another revolution. Where are the marks? If the marks on the shaft and prop have returned to their original position, then the transmission is 2:1.
If the prop and mark are not back at the top, rotate until the marks are again a the top and note the mark on the fly wheel. That will give you the ratio. Repeat for reverse.
On this forum there is lots of information for your particular boat. If you have original equipment then according to the specs for your boat you have a 5411 Universal Diesel. This unit has 11HP. The ratio of the Hurth transmission, it will be a 2:1 reduction. It should also have a 12x11 two blade prop. Now, out of personal interest I have been talking to some experts in the field of power requirements. Rule of thumb being about for every ton (2000 pounds) of displacement you should have 4HP . This makes my boat well under powered and it looks like yours, wether its a standard keel or with a centerboard, likewise. According to base data your displacement is anywhere from 9650 to 10,250, pending your style of keel. I include the link on this site to get any other information you might want to know. https://sbo.sailboatsowners.com/downloads/Oday_28_39127611.pdf
thanks for all the info . Yes the O Day is under powered , but to repower is costly and I'm not sure how many years I can physically still sail
the link doesn't work
So, my 10,250 (5 ton) O’Day 322 could use a 20 HP engine. My boat came with an 18 HP MAX, 16 HP Continuous rated Yanmar 2GM20F…so a little under powered I guess. I am fortunate to sail Lake Michigan, so no tidal current to worry about. But I wouldn’t really want anything smaller.
Greg
Directions to the information for a O'Day 28
>Go to Forums
>Scroll down to O'Day Bigger Boats
> go to Boat Info (left side of page)
>Select your Model
>Go to Downloads (left side of page)
>Go to Oday 28 Owners Manual
I've tried it twice, should work.
>Go to Forums
>Scroll down to O'Day Bigger Boats
> go to Boat Info (left side of page)
>Select your Model
>Go to Downloads (left side of page)
>Go to Oday 28 Owners Manual
I've tried it twice, should work.
I'm researching the web for fact based evidence, to let me make an informed decision if buying a fixed three blade prop (not willing to spend for a feathering or folding unit) would get me some of the missing power I would like to have, primarily to have somewhat better speed when motoring, but also some "umph" to help me get out of trouble should I need it. I understand the drag part when under sail, which I can accept as I don't race. My engine is a 16 hp and my displacement is 10'600. Going by rule of thumb I'm way underpowered. I'm not looking to be a speed boat, however an extra knot or two would be nice. If anyone of this forum has done the research or even just for the hell of it, changed their two blade for a three blade, I would appreciate your input, good, bad or indifferent as always. Thanks.
The maximum speed of your boat is determined by the hull shape more than the prop or the engine's horsepower. Secondary factors are current, sea state, wind, bottom condition and other environmental conditions. If you can get 6-6.5 knots in flat water, that is all you are going to get. A little extra hp would be nice in adverse conditions, like punching through waves and chop, but not all that much. The theoretical hull speed for an O'Day 30 is 6.75 knots or 7.77 mph. A 3 blade prop won't improve your hull speed above that amount and will slow your sailing speed, especially in lighter air.
Hull Speed Calculator — Optimal Vessel Speed
Hull speed calculator is a simple calculator that determines a vessel's hull speed based on the length of the vessel's waterline.
www.omnicalculator.com
This “rule of thumb” needs context to apply to a boat.
If all you need is a simple in and out of a Marina, day sailing in fun/moderate weather then a a small engine is likely all that is needed. I think Smart Car for grocery runs. Sure a 68 Shelby Cobra GT350 might be a blast to pull into the parking lot, but is it needed. Ok maybe.
My experience with a three blade suggests the type of 3blade, and the correct diameter plus pitch can yield improvements worth the expense.
In acquisition of the prop I discussed the problem with the prop designer at WestbyNorth. I gave him the engine, shaft, and transmission specs. I listened to his ideas. I asked about peak and cruising performance. We discussed WOT to deal with those emergency times. Now after installation and successful use cruising and powering through 40 knot wind squall in The BC gulf islands I’m very hap with the decision.
It was a big improvement under power, and I find the sailing performance only marginally impacted (less than 10%).
We had a 10hp Universal M12 in our 28 and the OEM 2 blade prop. We had the shoal draft keel with a displacement of about 7700 lbs. I never thought it was underpowered. In flat water and with a clean bottom we could easily go over 6 knots, close to the hull speed of 6.4. We didn’t have a tachometer but it still seemed like a we had a lot of throttle left. A fouled bottom would definitely cut into that by upwards of a knot, but that’s less expensive to fix than a new prop. Also remember that when going against current speed over ground drops but you’re still up against the same hull speed limit through the water.
For every one ton (2000lbs) the rule of thumb is 4 hp. This is how marine engineers get a rough estimate to size a cruising engine for a sailboat. For a day sail sure I have enough with 16 hp. I'm working on improvements for my 6-8 month cruise which is on my bucket list.
This piqued my interest, so I dove into my textbook on yacht design and engine selection, looking for guidance. I found scant evidence for the rule of thumb, there was however considerable discussion about prop shape and hull form with enough math and graphs to make one's eyes glaze over.
Practically, if the boat with a clean bottom and prop can approach hull speed in flat water at less than WOT, then it has enough power with the engine and the prop on the boat. Having extra power to plow though chop is a bit of a red herring. If the chop is severe enough to warrant a lot of extra power to reach hull speed, it is too rough to run at hull speed.