Advice on new boat buy please/ underpropped?

[Turtle Blues]

Yes Sir,
Rechecked to make sure I didn't transpose them.

 

[Portage_Bay]

I would take what that owner says with a large grain of salt.

owner claimed 2.5gph total at 7 knots.

which is 2.8 nautical miles per gallon. Maybe at just above idle, maybe. My experience with the hull form in your pics for a boat of that size and weight is limited to two boats. But I expect that to get to 7.5 - 8.0 kts will be closer to 1.0 - 1.2 nmpg.

 

[DavidM]

That is a little strange, but technically, I don’t see why not.

David

 

[Turtle Blues]

Yeah, I' not real good at close monitoring, my fuel consumption calculation for slow cruising is 1npg.

 

[rslifkin]

I would take what that owner says with a large grain of salt.

which is 2.8 nautical miles per gallon. Maybe at just above idle, maybe. My experience with the hull form in your pics for a boat of that size and weight is limited to two boats. But I expect that to get to 7.5 - 8.0 kts will be closer to 1.0 - 1.2 nmpg.

2.5+ nmpg at 7 kts is not unreasonable for a 41 footer. Above 7 it'll go down fast. At 6.5 - 7 kts I'm over 1 nmpg with gassers, diesels will do much better than lightly loaded large gas engines.

 

[O C Diver]

As a point of reference, Slow Hand (50' LOA, 42' WLL, 40K to 45K displacement) cruised:

6 knots 1.2 GPH 5 MPG
7 knots 2.0 GPH 3.5 MPG
8 knots 3.7 GPH 2.1 MPG

Single 4045 John Deere @ 135 HP
2.5:1 ZF gear
28 x 20 4 blade Michigan propeller

Ted

 

[Turtle Blues]

Ted,
Your numbers bring tears to my eyes. It makes extended cruising look affordable.

 

[O C Diver]

Ted,
Your numbers bring tears to my eyes. It makes extended cruising look affordable.

When fuel was over $6 per gallon, I did a lot of cruising at 6 to 6.5 knots. Nice to have a 3,000 mile range and tanker at $3.47 instead of $7.00 per gallon.

Ted

 

[Portage_Bay]

As a point of reference, Slow Hand (50' LOA, 42' WLL, 40K to 45K displacement) cruised:

6 knots 1.2 GPH 5 MPG
7 knots 2.0 GPH 3.5 MPG
8 knots 3.7 GPH 2.1 MPG

Single 4045 John Deere @ 135 HP
2.5:1 ZF gear
28 x 20 4 blade Michigan propeller

Ted

Wow! Remarkable efficiency. Wish I could see numbers like that.

 

[PNK]

I don't have the propulsion expertise of the posts above, but it sounds to me like while you really like this new boat, you want to be able to cruise comfortably at 8 knots, which I am guessing means not too much noise or vibration. The current owner may be aware of an issue that is preventing that, but you could simply let him know that this is a decision-maker for you, and you want to sea-trial the boat to see if it can do what you want. You will learn a lot from which answer he gives you, and whether you want to continue with the buying process.
Even if, with all this help, you are positive the boat isn't propped correctly, you won't know whether some other issue, or issues, are also involved, and what the cost to repair them would be.
Since you think the first buyers are backing out of the deal, that could be a red flag, and a reason to proceed cautiously.

 

[88FuHwa]

Your CHB 34 has a semi-displacement hull. It will lift up out of the water to a partial plane allowing increased speed.
It sounds like the 41 ft trawler has a full displacement hull. The hull shape is not designed to lift the bow. Maximum speed on it would be about 8 knots regardless of engine size, revs, or prop pitch. An FD hull travels through the water, not over the water. Stability in rough seas is usually better, but speed is limited by the hull shape.

Dear AusCan - I don't think the CBB 34 has a semi-displacement hull. I have a similar Fu Hwa 35 and is certainly not semi but is full displacement. It would take some significant HP to get this boat on plane.

 

[JimKane]

There are a number of good prop calculators online. Google “marine prop calculator “ for the information you are looking for.

 

[Northern Spy]

I'd rather buy an under propped boat that an over propped boat.

Sea trial it. See if it'll do WOT. Empirical data is worth more than theoretical calcs.

What's the big deal with getting 2 more inches of pitch beat into the props if necessary? Unless they're NiBrAl. Work it into the negotiation price if need be.

 

[BruceK]

Dear AusCan - I don't think the CBB 34 has a semi-displacement hull. I have a similar Fu Hwa 35 and is certainly not semi but is full displacement. It would take some significant HP to get this boat on plane.

What are the features of the CBB (?CHB) hull which cause you to class it as full displacement? Certainly agree an FD hull won`t plane, an SD may try to but with limited partial success, maybe more if overpowered. The CHB was sold in Australia,from where Auscan and I hail, as a Clipper 34 and is definitely, semi displacement.

 

[u4ea32]

Two things. Firstly, use a photo tach as too many engines do not have reliable tachs unless they’re newer digitals. Secondly, pay little heed to waterline length guesses and the fallible 1.4 multiplier X sq rootWL. I’d guess the 41 footer to have no more than a 1.2 multiplier if it indeed is FD.

As previously suggested insure full RPM test is done to assess current propping and engine health. Then, once the correct props are in place you can do a fuel burn vs RPM vs boat speed to establish your desired cruising regimen.

1.05 x sqrt waterline is nearly always the top end of linear increase in drag. Over that speed, drag increases exponentially.

1.4 x sort WL is often the WORST efficiency in nearly any normal hull, planing, semi planing, or full displacement. Only weirdly designed hulls with really odd bulbs and bustles are relatively efficient at that speed, and for certain none of us owns such a beast.

The reason (1.05 x sqrt WL measured in feet, speed measured in knots) is so specific is that is the speed where the bow wave and stern wave somewhat cancel out, so there is a local minimum, actually less drag than 1 x sqrt waterline, and far less than any faster speed.

Slow down!