Inboard diesel hull with an outboard, consumption/performance ?

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Hi guys posting on your forum from the UK
I am after some information on the above.
I am considering purchasing a new boat (hull only) that is normally supplied with an inboard diesel/leg but fitting an outboard on a pod with jack plate.
This is the hull http://www.aquafish-boats.co.uk/index_4.htm
It is a 23ft fast planing fishing boat weighing approx 1,400kgs or 3,000lbs.
With a 220hp inboard/diesel/leg the manufacturers claim around 35kts at WOT.
I am considering fitting a 225hp Honda outboard on a pod with jack plate instead.
The reasoning for this is that the inboard/leg costs ?24,000 fitted, only has a 2 year warranty, weighs 600kgs combined & costs ?1,000 per service :facepalm:
The 225hp Honda outboard on the other hand costs ?13,000 fitted, has a 6 year warranty, weighs 275kgs & costs around ?280 per service.
Also the outboard will mean that I do not have a big engine box on deck
My only concern is whether fuel costs with the outboard will make this prohibitive ?
I can"t find anyone here in the UK that has done this conversion & the hull makers claim that it will be to expensive to run compared to their inboard diesel option ?
I have done as much research as I can myself & think that with the outboard performance will be close to 40kts at WOT.
I also believe that at a cruise speed of around 4,000rpm speed will be around 24kts & consumption around 6.5 imperial gallons per hour (30 lph).
I can"t confirm my estimates this side of the pond as most boaters with this sort of hull run inboard diesels.
I know that some of you guys run similar weight planing hulls with big outboards so am hoping that my estimates of performance but more importantly consumption might be confirmed.
Diesel is obviously cheaper than petrol in the UK but as my useage is only around 150 hours per year I can"t see how the cost saving would be anywhere near the purchase price saving of the outboard & the considerably cheaper servicing costs

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(every day I live with fear, sometimes she lets me go fishing)

 

[aerobat]

Re: Inboard diesel hull with an outboard, consumption/performance ?

i have a 21 ft fishercabin- the eurostar 665fc with a 150 hp direct injected evinrude V6 on the back. the weight of the boat (with engine) is about 3200 lbs. top speed is about 36knots . cruising with about 25-28 knots results at about 35 litres / hour but the consumption explodes when you ride WOT. ( at least with the evinrude )

i think a diesel will be far more fuel efficient.

it has to been said that my boat was designed for an outboard, you will have to do a conversion at a boat designed for an inboard.

what about bying the hull and looking for a used inboard diesel and not the factory package? . volvo penta gives many options in the 200-250 hp power range in diesels and i think for the price of a brand new honda you will get a sweet light used penta diesel. the disadvantage is of course you will have not warranty on the engine.

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

I appreciate that a diesel inboard/leg will be more economical than a 4 stroke outboard but I am trying to confirm that the saving dosen"t justify the near double the purchase cost & nearly ?1,000pa dearer service costs.
All the conversion will entail is bolting a pod & jackplate onto the transom to allow the Honda to be fitted.
The builders have already confirmed that the transom is plenty strong enough to take the pod/outboard.
The inboard engine bay will house the fuel tanks & will be fitted with a flush sealed lid by the builders as opposed to a raised engine box.
I presently run a Raider 18ft Wheelhouse (about 1,500lbs hull) with a 115hp Honda 4 stroke EFI like this

this gives about 36kts at Wot & about 16lph (3.5 imperial gallons) at around 22kts & 3,600rpm.

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[aerobat]

Re: Inboard diesel hull with an outboard, consumption/performance ?

well, its pretty a mathematical calculation in your case. you save roughly 11000 ? choosing the outboard and ride 150 hours a year. assuming the diesel burns lets say 25 litres of (cheaper) fuel comparing to 35 litres on the outboard you save - depending on the fuel prices in uk- xxx ? per hour when operating the diesel. so, you can calculate how many running hours ( and years when you run 150 hours a year) it will take until you use up the 11000 ? you saved at installation. when the calculation gives you several years of boating until it equals and you prefer the outboard simply go for the honda !

 

[Silvertip]

Re: Inboard diesel hull with an outboard, consumption/performance ?

I know you are considering Honda power. However, the Yamaha web site has a "Performance Bulletins" page that allows you to research any of the their engines (two or four stroke) on a very big selection of boats. Boat speed at idle through WOT, speed, fuel consumption, the prop used for the test and in some cases, noise numbers are provided. I suggest you check that site, select the engine and then select a boat of the same or similar size and weight. fuel consumption will be listed in GPH (gallons/hour) as well as MPG (miles per gallon) so have your calculator handy for the conversions. That said, at WOT your 225 is going to consume about 10% of the engines rated HP. Yes -- that is 22.5 gallons per hour. It will be somewhat less obviously at a little over half to about 2/3 throttle. Power settings higher than that will result in a sharp increase in fuel consumption. Honda, Yamaha, Mercury will not make a significant difference in fuel consumption.

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Hi Silvertip & thanks for your reply.
I appreciate that at WOT the outboard will use a lot of fuel but I have no intention of using anything near WOT revs for 99.9% of the time.
My present 115 would use around 11.5 GPH at WOT but at 3,600rpm it uses around 3.5 GPH so around 30%.
Based on that I would assume that a 225 at a cruise spped of 4,000rpm max would use around 30% of that 22.5GPH figure or just under 7 GPH ?
Do you have a link to the Yam "performance bulletins" as I can"t find them on the Yam UK site ?

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[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Hi aerobat, that is my thinking exactly !
Here in the UK for fishing boats, & certainly any over about 20ft then diesel inboard is far far more popular than outboards.
4 stroke (& 2 stroke) EFI outboards have come on leaps & bounds in terms of consumption & reliablity IMO & with a fast planing lightish boat I personally cannot see any advantage in the inboard diesel unless you are doing an awful lot ot of hours ie. commercial use.
I am just hoping to find out some consumption figures from any of you guys running similar sized motors & weight hulls as it is difficult to get any figures from this side of the pond.

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[aerobat]

Re: Inboard diesel hull with an outboard, consumption/performance ?

I am just hoping to find out some consumption figures from any of you guys running similar sized motors & weight hulls as it is difficult to get any figures from this side of the pond.

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hi headlight !

like said my 21ft boat with a 150 hp uses roughly 9-10 US gallons / hour ( 35 litres) at 4000rpm which results in roughly 26-28 knots. . your boat, wich is a little bigger and has a bigger engine will use probably about 11 usg at this load.

silvertip gave a good hint- looked at it and found this test :

http://www.yamaha-motor.com/assets/...ulletin_4stroke_hpv6_al_hyd-230wa-f225txr.pdf

you can see the 4 stroke 225 yamaha at 11-12 us gallons reving 4000 rpm.

when you give fuel consumptionsof your 115 honda in gallons coming from uk- are you talking US gallons or IMPERIAL gallons ??? thats a difference.

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Thanks for that link aerobat.
That 230 WA is probably going to be quite close to the set up I am looking at.
At 4,000rpm it is showing 10.5 GPH US gallons.
That would be 10.5 x .8327 = 8.74 imperial gallons or nearly 40 lph.
The speed given is 25.5 mph so that is roughly 1.5l per mile.
Those figures are a little more than my guestimate of 7 GPH but liveable.

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[aerobat]

Re: Inboard diesel hull with an outboard, consumption/performance ?

yes, calculating 40 liter / hour at this cruising speed will match you very close to reality what you can expect from your outboard project.

i also found this cummins mercruiser 220 hp diesel datasheet with fuel comsumption.

since 4000 rpm at the honda is roughly 75% revs this would equal about 3000 rpm at this diesel ( max rpm 3800) .

you can see the diesel is stated with 24.2 liters/ hour at this load .

calculate 15 litres / hour difference between a diesel and the outboard and you can see how you will go with the savings at installation.

what fuel prices do you currently have in uk ?

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Pump prices are around ?1.40 per litre diesel & about ?1.33 per litre petrol.
Marina diesel prices are less but if I am trailing then I have to pay petrol station prices.
I think that I will be able to get a little better consumption than the links figures.
Firstly my all up weight will be a little less than the 230WA they have used, also the hull I am looking at has more lift than a 230 WA so should need slightly less revs to maintain comparable speeds.
Add to that a stainless as opposed to ally prop as in their test & I think I will be doing 2-300rpm less.
35 lph should be doable IMO.
My experience of Honda"s is that their consumption is very very good.

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[aerobat]

Re: Inboard diesel hull with an outboard, consumption/performance ?

so- its interesting to calculate this

at the diesel we have 25 liters x 1.4 ? = 35 ? fuel costs / hour , at the outboard 40 x 1.33 = 53 ? fuel costs / hour

so the diesel saves 18? / hour but the outboard 11000 ? at installation.

this means you have 611 ( 11000:18) hours or 4 years ( 611:150 with 150 hours a year) until your 11000 ? saving is used up.

when planned to keep the boat more than 4 years the diesel is a net win comparing today fuel costs. remember that when the fuel prices rise the diesel will benefit earlier.

this calculation does not include warranty/ resale / maintanace aspects of course, just the pure fuel costs.

i know its not what you want to hear- but when you can make the extra investment today i would surely go for the inboard diesel. at rising and rising fuel prices in europe the engine cannot be efficient enough, in 4 years the diesel installation will give you a net benefit, up from the first hour a better range, easier to organize fuel at the water and most probably a far better resale of the boat some time.

but boating is not only money calculation , you must like it. when you feel better with the honda simply go for it !

 

[aerobat]

Re: Inboard diesel hull with an outboard, consumption/performance ?

so- its interesting to calculate this

at the diesel we have 25 liters x 1.4 ? = 35 ? fuel costs / hour , at the outboard 40 x 1.33 = 53 ? fuel costs / hour

so the diesel saves 18? / hour but the outboard 11000 ? at installation.

this means you have 611 ( 11000:18) hours or 4 years ( 611:150 with 150 hours a year) until your 11000 ? saving is used up.

when planned to keep the boat more than 4 years the diesel is a net win comparing today fuel costs. remember that when the fuel prices rise the diesel will benefit earlier.

this calculation does not include warranty/ resale / maintanace aspects of course, just the pure fuel costs.

i know its not what you want to hear- but when you can make the extra investment today i would surely go for the inboard diesel. at rising and rising fuel prices in europe the engine cannot be efficient enough, in 4 years the diesel installation will give you a net benefit, up from the first hour a better range, easier to organize fuel at the water and most probably a far better resale of the boat some time.

but boating is not only money calculation , you must like it. when you feel better with the honda simply go for it !

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

so- its interesting to calculate this

at the diesel we have 25 liters x 1.4 ? = 35 ? fuel costs / hour , at the outboard 40 x 1.33 = 53 ? fuel costs / hour

so the diesel saves 18? / hour but the outboard 11000 ? at installation.

this means you have 611 ( 11000:18) hours or 4 years ( 611:150 with 150 hours a year) until your 11000 ? saving is used up.

when planned to keep the boat more than 4 years the diesel is a net win comparing today fuel costs. remember that when the fuel prices rise the diesel will benefit earlier.

this calculation does not include warranty/ resale / maintanace aspects of course, just the pure fuel costs.

i know its not what you want to hear- but when you can make the extra investment today i would surely go for the inboard diesel. at rising and rising fuel prices in europe the engine cannot be efficient enough, in 4 years the diesel installation will give you a net benefit, up from the first hour a better range, easier to organize fuel at the water and most probably a far better resale of the boat some time.

but boating is not only money calculation , you must like it. when you feel better with the honda simply go for it !

Plus the servicing is nearly ?1,000pa cheaper so that is 55 hours or a third of a years fuel paid for as well.
Although resale value will be more with the diesel as the outboard boat is ?11,000 cheaper then it"s resale price can be ?11,000 cheaper also.
I still think that the consumption figure will be closer to 30lph rather than 40lph (I hope).
Still hoping for some of you guys running a modern 225hp 4 stroke EFI on a 23ft"ish & 3,000lb"ish hull to post some consumption figures.
As a rough guestimate I am looking at a boat twice the weight of my present set up with twice the power.
At worst cruise consumption should be no worse than twice my present boat (I would think) & although the new boat would be twice the weight it is not twice the size nor have twice the drag.
My local Honda dealer reckons about 1 litre per NM at cruise of around 3,800 for the set up (but obviously can"t give any guarantees) & the boat builder reckons 3 times that (but he does want to fit a ?24,000 engine rather than sell me just a hull) !

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[Silvertip]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Here is the link the main Performance Bulletins page on the Yamaha web site. Accurate fuel consumption data cannot be easily determined unless the vessel is equipped with a fuel flow monitor and that's why I like Yamahas test page. Other manufacturers are doing this but none of them have anywhere near the number of boats and engine tests as they do. The WOT rule of thumb is usually used as a reference only because it doesn't matter whose engine you are testing -- it will burn 10% of its rated HP at WOT (or very close to it). That's why I recommended the Yamaha web site. The link below will allow you see the variety of boats on which a 225 HP engine (two or four stroke) was installed and tested. I understand you won't be running wide open most of the time -- most of us don't.

http://www.yamaha-motor.com/outboard/products/perfbull.aspx

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Here is the link the main Performance Bulletins page on the Yamaha web site. Accurate fuel consumption data cannot be easily determined unless the vessel is equipped with a fuel flow monitor and that's why I like Yamahas test page. Other manufacturers are doing this but none of them have anywhere near the number of boats and engine tests as they do. The WOT rule of thumb is usually used as a reference only because it doesn't matter whose engine you are testing -- it will burn 10% of its rated HP at WOT (or very close to it). That's why I recommended the Yamaha web site. The link below will allow you see the variety of boats on which a 225 HP engine (two or four stroke) was installed and tested. I understand you won't be running wide open most of the time -- most of us don't.

http://www.yamaha-motor.com/outboard/products/perfbull.aspx

Thanks for the link Silvertip.
I have had a good look at the figures armed with a calculator to do the conversions.
It would seem that I am not going to be far off in my original guestimates of consumption & performance.
The boat builders suggested ouboard consumption is as I thought way way over the top.
I have now actually found someone here in the UK that is running the particular hull with an outboard.
He is running an Optimax 150hp & claims 1.5 litres per NM consumption at 22kts cruise or 33 lph which is 7.3 imperial GPH or 8.7 US GPH.
His 150hp will be working harder to maintain that cruise speed than my 225 would be so I am pretty sure that my consumption would be no worse than his at the same speed & possibly slightly better.
His boat is set up exactly as I want & is for sale but unforunately I don"t fancy the 150 Optimax as a power plant & he won"t split the package at a price that I think is reasonable.
I think that after xmas I will go ahead & order the hull !
Thanks for all the help guys.

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[Silvertip]

Re: Inboard diesel hull with an outboard, consumption/performance ?

One thought about your comparison of the Honda 225 with the Optimax 150. While the power difference certainly is a factor, when comparing a two stroke to a four stroke with the same power, a two stroke will almost always power an identical boat faster at any given rpm. So rather than comparing fuel consumption at a given rpm, take a good look at the charts you are comparing and instead concentrate on speed, and what the rpm is at that speed. A two stroke will almost always be at a disadvantage when comparing rpm. But since the engine pushes a boat faster (generally) than a four stroke, speed comparison is a more accurate comparison. Yamahas web site and my own fuel consumption studies (although those were done an 75 HP engines with a flow monitor) verify that.

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

One thought about your comparison of the Honda 225 with the Optimax 150. While the power difference certainly is a factor, when comparing a two stroke to a four stroke with the same power, a two stroke will almost always power an identical boat faster at any given rpm. So rather than comparing fuel consumption at a given rpm, take a good look at the charts you are comparing and instead concentrate on speed, and what the rpm is at that speed. A two stroke will almost always be at a disadvantage when comparing rpm. But since the engine pushes a boat faster (generally) than a four stroke, speed comparison is a more accurate comparison. Yamahas web site and my own fuel consumption studies (although those were done an 75 HP engines with a flow monitor) verify that.

Fully agree with you there Silvertip.
I have had many 2 strokes & my present 115 Honda is only my second 4 stroke.
What I have found over this side of the pond is that when comparing modern EFI 2 strokes such as the Optimax & ETEC against EFI 4 strokes of the same HP the consumption is very similar for a given speed on the same hull, although as you say the 2 stroke will be doing less revs for the same speed.
I personally think that 150HP is a little low power wise for the hull I am looking at, plus I am not a fan of Optimax"s.
In inboard diesel configuration the hull is supplied with HP"s from 120 -270 (max) & the best compromise between consumption & performance seems to be between 190-240hp.
I think the 225 outboard will give similar performance (not consumption) if not better to the 240 inboard due to it being half the weight plus the prop running in cleaner water due to the pod mounting & the fact that the optimum running height can be set due to the jack plate.
This side of the pond all L/S motors are set on the transom & you will not see one being run higher.
When I re-engined my present boat I could not get the revs I wanted & was disappointed with performance.
The norm in the UK would be to reduce prop pitch to bring the revs up & indeed i did do this but was still dissapointed.
I then spoke to a Honda race mechanic about my problem & after explaining what was happening he said to raise the engine up off the transom.
2 transom clamp holes & 38mm (1 1/2" inches) lifting later my performance & consumption were spot on which is one of the reasons why I will be having a jack plate (something you never see over here) on my new boat.

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[Silvertip]

Re: Inboard diesel hull with an outboard, consumption/performance ?

You have apparently discovered that setup with an outboard involves more than just bolting it on and then hunting for the correct prop. Engine height is indeed an important factor when attempting to get the most out of combination. I would suggest you install a water pressure gauge on the Honda for the simple reason that it allows you to determine immediately if you have raised the engine too high and are starving the water pump. It is also a good indicator of impeller condition. Note the idle, mid-range and WOT WP readings with a new impeller. Over time, any significant deviation from those readings indicates water pump service is needed.
Good luck with your choices.

 

[headlight]

Re: Inboard diesel hull with an outboard, consumption/performance ?

Lifting engines is virtually unheard of this side of the pond Silvertip !
All dealers here will tell you that the hull requires a L/S & that is it. You then pitch the prop to get the correct revs.
I am a member of a large fishing forum here in the UK & I was advised by members of the boat fishing section (some of whom were boat dealers) that I shouldn"t raise the engine as I had the correct length shaft for the hull & all I should be doing is altering prop pitch.
After my success with lifting my engine about 5 or 6 members with the same hull as me have raised their engines with the same positive results as me.
My engine has been running in it"s raised position for nearly 3 years from new now so if I was going to have any cooling issues they would have shown by now.
The impeller is changed annually as part of the warranty service & has shown no signs of wear.
Even raised 1 1/2" my anti ventillation plate is still within 1/2" of the bottom of my hull.
I may install a WP guage on my new boat though as I may be altering engine height according to boat load & water conditions.

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