Diesel Questions

While I'm an old gummer who has boated for 50+ years I'm moving toward taking the leap into a diesel powered boat for the first time. My unfamiliarity with diesels brought me to this forum where I've been lurking for awhile.

My questions revolve around the hours on a diesel, engine selection, etc. By way of background information, I'm looking for a boat in the 50'-55' range with twin engines. I'd like a boat that will easily cruise at hull speed but will be able to plane at 12+/- kts. The boat will be well equipped, have upper and lower helms and be capable of supporting a couple living aboard for 3-4 months at a time.

Question #1, I read a post on here where the gist of what I read suggested that knowing the hours on the engines isn't as important as knowing how much fuel has gone through each engine. In other words, how hard has each engine been run during its life.

How difficult is it to find out that information? Is there a way to hook an engine diagnostic tool to the engine that will give that information? How does one go about finding that information if the owner/seller has not kept those records?

Question #2 has to do with fuel economy. Assume for a minute I'm looking at two boats. Further assume that both boats are similar size (52'-54') and weight (40,000-45,000 pounds) and hull design. Both capable of planing speeds in the 12kt range but both mainly cruised at hull speed with short daily runs (30 minutes) on plane to bring the engines up to operating temp.

If one boat has engines in the 400-450 horsepower range and the other has engines in the 575-625 horsepower range, what would you expect the fuel burn rates to be on each of these boats?

Question #3 If you were in my position (and given this brief description of the boats) which would you likely prefer, the higher horsepower boat or the lower horsepower boat?


Thanks in advance for your time and for sharing your considerable expertise. I appreciate your help.

GFC

Thanks for the input, I appreciate it.

So, when one is considering a used boat, how do you know what the HP needs are? I'm looking at Ocean Alexanders in the 52'-54' range, around 1990 vintage. They have engines whose HP range runs all the way from 375 on the low end to over 500 on the high end. 400-425HP seems to give a cruise speed of 12-15kts.

Another make I looked at was a 52' Westbay equipped with twin 680hp Man's. The Westbay is wider (17'1" beam vs 15'6" on the OA's) but would it really need that much HP to move it at the listed cruise speed of 24kts. That would appear to be a very inefficient hull from a such well respected designer, or am I missing something.

Any thoughts on either of these two makes or their engine selections?

OK Carl, I was doing fine through the first three lines, then ya lost me. I understand the difference in fuel burn between planing at 12-15kts and running at 20-25. (Though on my current boat, a 330 Sundancer, once I'm on plane I'm very close to .97-1.03mpg from 3400rpm's all the way up to about 4200rpm's. At that point the carb secondaries open and the fuel burn rate increases, dropping the mpg.)

Let's assume the Westbay does plane at 12-14. What do you mean by "ride well" at 24?

Are you talking about the boat riding on top of the water in a more bow-down attitude and not plowing through the water?

If both boats (Westbay and Ocean Alexander) could plane at 14-15kts, what would you expect the fuel burn difference between the two, given the engine difference? Would there be a significant difference, or would the difference be minor because the larger engines in the Westbay don't have to work as hard to maintain that speed?

K1W1 and Highlander,

Thanks for getting back on this one. I do appreciate your time and knowledge. First, no naval architects out here. There are some in the Seattle area, but not on this side of the state.

Both the OA and Westbay are planing hulls. I have not looked up the specs for entry angle and deadrise, but am very familiar with those terms and the impact each has on how the boat rides when the water gets bumpy.

Most of the OA's in YachtWorld have the weights listed, but that 52' Westbay does not. I'm pretty sure I can find that out from Westbay, as well as the deadrise.

Here's the specs on a new 54' Westbay...
SPECIFICATIONS
Length Overall 54’
Centerline Hull Length 50’
Waterline Length 42’ 8”
Beam 16’ 2”
Deadrise Midship 20º
Deadrise Transom 17º
Draft 3’10”
Displacement (approx) 54,000lbs

By way of comparison, the 52' has a beam of 17'1", 5' draft and LOA of 57'6". I know what assumptions can do, but one could assume that the 52' boat, given the wider beam and deeper draft that it has deadrise numbers similar to the new 54'.

There's an OA Rendevous next month that we're going to attend. That will give us a chance to crawl around on some of them to take a closer look and talk with the owners to get their impressions. That's probably the best way for us to look at several boats in one area at the same time.

Thanks Cheerful Kelly,

Great place to be. I've been lurking and doing a lot of reading. Very interesting and a somewhat different viewpoint from BABC.

BTW, I'll be in your neighborhood next week. Sunday we're headed down and will be there a week. I'll see if I can figure out how to PM you with my cell number. Are you up for another lunch?