Atomic Four Prop

[ArtJ]

My Tartan 34c is 34.5 feet and 11,500 lbs . The Indigo is able to push
the boat at least 5 knots in a nasty chop when running rpm at
2200 to 2400 rpm.

What rpm are you able to achieve? Maybe the distributor may need
to be turned. Is the bottom clean?

[ghaegele]

I've tuned the distributor to maximum RPM at cruising speed and can get about 2200 max. The boat gets hauled every year and the bottom never gets very bad. I like the Indigo but sometimes feel like I got better drive from a larger diameter two-blade that I had on years ago. But that could just be a selective memory.

Art, I'd say your Tartan is close in specs to my C&C. (Actually, I have 35.5 feet and 13,800 displacement. But that's not a huge difference) Five knots in a chop, maybe. But not in wind and sea. I sail in the LI Sound where we often get more than a chop. I might be expecting too much from my engine, but I feel like going to power is just not an option when I am heading into significant wind and sea.

[ArtJ]

I just took a look at the Michigan propeller that was on the Tartan when
I bought it I believe it was a 12 X9 . I was able to get 6 knots
in calm seas at 1700 rpm. But the boat would lug in a steep short chop.
and only do around 3 knots.
I changed both pitch and diameter, reducing both. The current diameter
is around 10 inches. It made little difference in rpm.

I then went to the Indigo.

I have sailed Buzzards bay often, including Long Island sound.
Even in rough seas off woods hole I could do 5 knots with the Indigo.

If you are talking about heading into significant seas with a lot of current
against you, you may be losing headway due to current. There is always
some conditions which will try any combination.

Tom Stevens experimented with the propeller on his Tartan 34C
and found the best thrust combination was with the current design.
I know that he has made some minor improvements to the prop.
You might want to check with him at Indigo and see if your prop
has the latest "twists". Tom upgraded my prop at no charge.
I doubt that this would solve your issue. I think you should
speak to Tom Stevens because a larger diameter prop will give
you less thrust, not more.

Regards,

Art

[ghaegele]

My prop is probably more than 10 years old. I think I will give Indigo a call and see if he has any suggestions. (And no, Art, I'm not talking about heading into current. That's silly.)

[Mark S]

Art,

Do you have direct drive or reduced drive? My boat is 35 feet and weighs 13,000 lbs. and I can't get above 1900 rpm with a direct driven Indigo. With the previous 13x8 I couldn't get above 1500 rpm. I'm beginning to think that the direct drive A4 is simply not strong enough to push boats this size.

Mark

[ArtJ]

I have a direct drive. If your engine is in reasonable shape the
timing can be set to produce 2400, or close as possible.
Note it must be set
at the rpm you desire in order to achieve the max or near max rpm.
This is set by running your boat flat out open in forward, or as close as possible and then turning the distributor for max rpm.
You may wish a compromise between flat out and your usual cruise
speed rpm.

[ghaegele]

I agree that we're underpowered. I set it at my cruising speed, which is just about 2100 RPM. I rebuilt my engine (direct drive also) two years ago and have under 100 hrs on it. Still I don't get much beyond 2200 RMP. (Perhaps its still breaking in?) I have played with the timing and don't think that I'm going to get much more RPM by adjusting it at full-out.

[ArtJ]

I should add the Tartan 34C, Intrepid inspired design, has only a 10 ft 4 inch
approx. beam which may make it easier to push to weather than a beamier
12 foot plus vessel.

[ArtJ]

I agree that we're underpowered. I set it at my cruising speed, which is just about 2100 RPM. I rebuilt my engine (direct drive also) two years ago and have under 100 hrs on it. Still I don't get much beyond 2200 RMP. (Perhaps its still breaking in?) I have played with the timing and don't think that I'm going to get much more RPM by adjusting it at full-out.

IF you are getting 2200 rpm and the bottom and prop are clean, you are
getting most of what the atomic 4 can produce for horsepower, the other
200 rpm won't change things that much.

[ArtJ]

When the going gets really tough, I tend to "tack" upwind, allowing the
sail (if hoisted) to assist the engine rather than take it totally on the nose.

[Mark S]

This is set by running your boat flat out open in forward, or as close as possible and then turning the distributor for max rpm.

You mean to advance the throttle beyond where the engine's current max rpm are achieved and then set the timing, noting whether the engine speed increases or decreases? You know, I've never fiddled with the timing. I've simply assumed that it was correctly timed by the people who maintained it for the prior owner. This will be a good experiment for next spring.

Mark

[ArtJ]

You mean to advance the throttle beyond where the engine's current max rpm are achieved and then set the timing, noting whether the engine speed increases or decreases? You know, I've never fiddled with the timing. I've simply assumed that it was correctly timed by the people who maintained it for the prior owner. This will be a good experiment for next spring.

Mark

Make relatively small changes, noting the previous position in case you wish
to back out. Do not make drastic changes, i.e. changes more than 1 inch
in either direction would be drastic. If the engine stalls. backfires, or pings
excessively, you know to back off a little.

[ArtJ]

You don't need to set the throttle beyond the point of max rpm, just
play with timing to get more rpm for that throttle setting.

[sastanley]

When the going gets really tough, I tend to "tack" upwind, allowing the
sail (if hoisted) to assist the engine rather than take it totally on the nose.

Bingo!!!

Art, thanks for mentioning this...it is key. Going straight into it is always slow..A displacement boat will always make better time sailing around the waves...Even a deeply reefed mainsail will help in these conditions. You don't have to go real far off..If motor sailing, pull the traveler up to windward and the mainsail will often fill and draw at as little as 10 degrees apparent.

[smaarch]

just a quick question: do you folks have tachometers on your boats?
i have no idea what rpm we are turning....so maybe this is a good idea.