What size engine to use in a TJason 35?

[Don Zink]

I am planning on having a 35 foot TJason built I was hoping the guys on here could help me out.
I am trying to decide which engine to use. The QSL9 450 or the QSC 8.3 500. Also what gear would be a good match
The boat will be a cruiser/ fishing boat but mostly used for cruising, with the hopes of doing the great loop and to take it to the Bahamas for extended cruises.
The TJason is a lobster boat
I would like a solid 18 knot cruise.
The boat will be a composite build no wood.
Best guess on the weight is 18-20000 lbs.
Northern lights generator 6kw.
350-400 gallons of fuel.
80 gallons of water
28 gallon holding tank.
bow thruster.
Galley up, convertible dinette
2 ACs
Thank you for the help

Topic

[Don Zink]

Read the Article.

Tony,
Thank you for your help. I did not understand the performance curves and was reading them wrong. After reading the article you directed me to I get it now..
Thanks again.
Don

[Tony Athens]

Understanding Cummins Power Curves

Don,

I need to find out of you really want to understand what you are trying to “wrap your head around”…………..So here goes.

#1—— You have not read or you do not understand the most basic principles as to what it takes to move a boat forward———– https://www.sbmar.com/featured-article/propellers-move-boats-engines-just-turn-them/

#2—You need to learn how to read the Cummins power curves—Look only at propeller demand curve –You are looking at chart for FULL THROTTLE maximum power curve. IMO, they are purposely mis-leading that way.

Apples for Apples with all engines propped per Cummins Guidelines
QSC 500 @ 2200 has approx. 300 HP available for cruising for hours on end
QSC 600 @ 2200 has approx.. 250 HP available for cruising for hours on end
QSC600 @ 2450 has approx.. 300 HP available for cruising for hours on end
QSL9 450 @ 1850 has approx.. 315 HP available for cruising for hours on end

Overall, HP for HP used , the QSL9 has a slightly better BSFC- Measurable ? I doubt it unless you run 2000+ hours per year.

Ease of maintenance—The QSL9 would shine big-time
NVA—I believe the QSL 9 may win that is mounted correctly.

Cost to buy—The QSL9 is less but the gear may cost more compared to the QSC 500.. The QSC 600 would be the most expensive option..

Study all 3 graphs until you “SEE” how to really read the HP that is available for cruising when propped correctly—It’s all there, they just “kinda hide it”

Tony

1 user thanked author for this post.

Graham Smith

[Scotth]

Are you looking at the propeller demand curves or the full throttle curves? Propeller demand curve is probably the better comparison and will make your numbers easier to interpret.

[Don Zink]

TJ 35 performance fuel curve

Tony,
I’ve been comparing the performance curves between 3 engines. I”m having trouble rapping my head around it.
What I’m having trouble with is when I compare the cruise rpm The QSC 500 at 2200rpm has 490hp with 1170ft-lb torque@ 15.7 gph
The QSL 450 at 1800rpm has 407 hp with 1188 ft-lb torque@ 14.5 gph.
Wouldn’t the QSC 500 push the boat easier because of the more Hp?

If I go with the QSC 600 at 2200 rpm 534HP with 1274lb torque @13.3 gph and ran it at 2200 would I achieve the same cruise with less load on the engine?
Another question I have is would the QSC be a smoother running engine at lower rpm compared to The QSL9?

Thanks
Don

[Tony Athens]

You would not want a 2:1 gear with the QSL9.. IMO, the MG5065A is on the light side for the QSL9. The 286A 1.75:1 would be my choice.

Tony

[Don Zink]

Hey Rob, I have to have it built to my specs….LOL I still have the Albin if all goes as planned I will put it up for sale in the fall. We will definitely be at the beach hopefully alot and we will be heading to Essex but not for the rendezvous bad timing …

[Don Zink]

Tony thank you for the reply.
The TJ can swing a 28iinch wheel with a 2 inch shaft.
the suggested gear for the QSC 500 is TD 5065 A ratio 2:04
for the QSL9 the suggested gear is also the TD 5065 A 2:04 or a ZF 286 1:75

[Rob Schepis]

Hey Don, welcome aboard. Having met you I have to ask – does the 35 have the headroom you will need? Or will it need to be modified per “your” specs?

Are you “boat-less” now? or will I see you at the beach again? Maybe Essex?

[Tony Athens]

TJ 35

With a 20000 lbs “working weight” you’ll be using about 18 GPH to cruise at 18 kts.. If heavier you’ need to burn more..If lighter, you’ll burn less

Basically, that translates into about 340-360 HP being extracted from the engine for cruising at these speeds at that weight.. So think 1:1 as to range at that weight and speed.. Not sure what you expect or are being told..

Look over the fuel/power curves for each engine and then you decide what would fit your profile best.

The QSL9 is a way simpler engine to deal with but will require heavier running gear. The QSC is a more complex engine, could be easily rated to 600HP, and will yield a higher WOT speed, but does not have a heavy “duty cycle” rating like the QSL9 does.

Both engines are 100% rock solid when “set up right”–That is the long term key.

Do you know what is the maximum diameter of prop your builder uses on these vessels? Shaft Diameter? Gear model and ratio?

Tony

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