Capt Tom, you and OMB may have more experience in these little boats. My experience is with tugs. I'm not sure of how you fellas run a fishing boat.
We have 7:1 gear reduction on most tugs because we are swinging a 80inch prop.... it's a whole nother thing.
Well they basically operate the same, I own a small un-traditional tug company. Can't really call it a tug, but my largest vessel is 40', my smallest is 24', both rigged for towing and salvage. The principals for both yours, mine and this fishing boat are pretty similar.
It's all about torque! Not HP. We're puller/haulers, not runners.
We must ask, how do we build torque to the wheels of a truck or to the prop of a boat.
Since we're working with zero information on this fellas boat, I've made some assumptions based on 30 + years working at sea aboard Navy ships and boats, commercial fishing boats, rescue boats, salvage craft, pleasure craft from 8' to 300', etc. Now none of that experience tells me what he has, however, based on the limited info given, I can make some assumptions, like I listed early on.
First and foremost is, he has a single 6.9l diesel engine. Well, what could he power with that thing?? I'm guessing something in the range of 28' - 34'.... possibly 36', but even 34' is pushing the limits of any performance of a single 170 HP N/A engine with zero guts. A Ford Lehman Marine diesel at 180 HP only pushes a 34' trawler (recreational vessel) at a good 10-12 knots full throttle on a good day. We're talking about a fella with the additional weight of working gear such as pumps, heavy davits, possibly a net reel, etc. He isn't going anywhere fast!!! He needs torque!
The next assumption I have to make is, it is a displacement or non-planing hull. No one in their right mind would try to plane a boat with a 6.9l IDI without some serious dragster equipment attached to it, no weight aboard... basically planing a piece of plywood.
If my assumptions are correct, then we have a starting point.
As I listed previously, based on shrimpers limited comments and guesstimations, there is an order to fixing his dilemma, starting with his rack. Once the engine is running well, we can figure out what he needs torque-wise and build from there.
Who knows, the engine itself may be unhealthy for various reasons, including very dirty cylinders and oil. I picked up a 77' Magnum with twin 16cyl DDEC's from Acapulco one year. The oil was so sludged up, we couldn't get the boat over 13 knots at full throttle, when she should have been doing 42 knots...this after an oil change. We changed the oil four times before San Diego. With each oil change, we gained enough speed to be able to do 28 knots entering San Diego. Between blowing out the soot on the top end and working out the old junk from the oil pan, we managed to free her up. Being that it was a DDEC, we didn't know we had 5 blown cylinders too. The computer cut out the bad cylinders, so we didn't have any indicators other than power losses. 3 cyl in one engine, 2 cyl in the other, totaled 5. The reader for the DDEC was missing and limited tools prevented our testing.... this was a bit of an emergency rescue mission if you will!
If shrimpers fuel issues as well as exhaust issues aren't dealt with, the rest is moot. Nothing else done to the engine will do a whit for him.
Plywood had some good prop points too. If you notice Deeproots transmission on his tug with prop combo, they are both huge!!!That is, he has a very large transmission...7:1, coupled to a very large prop, 80"! That's a 6.6' propeller!! Taller than most men.
Well his tug is pulling dead weight, at least at the start. So we ask, how do we pull dead weight??
With big transmissions that don't turn props fast at all, but the engines are spooling like heck to get to the best "torque" rpm. Peak torque is where you want your engine to be running at or there-abouts.
A speed boat with its little props, their transmissions are at or near 1:1. This means for every revolution the engine spins, so does the propeller. On Deeproots tug, his engine spins 7 times before the propeller turns once.
Also, his prop is a power prop. It is a huge 4 or 5 blade prop(possibly 7 blades) designed to give some slip so he doesn't lug or break lines, as well as the ability to push a ton of water with every revolution.
Our shrimpers boat should be similar for his size, maybe only 3 bladed, but built for pulling, not your typical yachtie prop, certainly not a speed blade...remember he's lugging, his words. Another assumption on my part, his prop is too big or his tranny's too small, or both, for what he's doing.
One thing I'd like explained to me, I didn't know is, plywoods great point of interest regarding "the diesels in boats is that they actually have a correction in the timing curve for load".
Could you tell us how this works or its principals??
I woke up to some pretty kewl stuff here this morning... keep it coming.
...... I am making an assumption based of the vessel size and engine chosen by someone, that the transmission isn't big enough to spool up the rpm to 1600 under load, even with a healthy engine. I think his tranny is too small, closer to a 1:1 ration than a 4:1 ratio. Under load, this too will cause lugging, even with a healthy engine.
