23 Conquest Fuel Tank Replacement

[SeaTrek2]

Give me recommendations and [in particular] local Miami, Florida recommendations for experienced people or firms I can reach out to for [a replacement fuel tank for a 2000 23 CONQUEST]. Thanks.

[Should the replacement tank be] made with aluminum or plastic polyethylene?

[Should the replacement tank capacity be the same as the original or less]?


BACKSTORY: I am the planning to replace the fuel tank, as I recently encountered some fuel in the bilge. About four years ago (in c.2016) when the boat was re-powered with Yamaha 150-HP engines, I had the fuel removed and the tank inspected. I was told the tank condition was fine. I am the original owner, and boat has always been meticulously cared for and maintained. I am guessing [the fuel tank leak] may be due to pinholes in the 20-year-old tank.

The current fuel capacity is 154-gallons. A fellow boater suggested [the replacement tank should have] less capacity [to promote] better fuel economy and performance. My boating is in Miami, Florida, and the farthest distance I go is the keys, but I want the option to go to Bimini.

[jimh]

The current fuel capacity is 154-gallons. A fellow boater suggested [the replacement tank should have] less capacity [to promote] better fuel economy and performance.

Regarding the capacity of the replacement fuel tank and whether it should be the same volume (154-gallons) as the original tank or lower volume: an assessment of the benefit that will occur must be made on the basis of how less fuel capacity will affect fuel endurance and range, and also on how less fuel weight will affect performance and fuel economy,

With modern fuel-efficient twin 150-HP engines the average fuel consumption is going to be about 8-GPH as a rough estimate. With 154-gallons of fuel we consider 140-gallons as useable consumption. This gives the boat an endurance of 140-gallons/8-GPH = 17.5-hours.

At full throttle modern fuel-efficient twin 150-HP engines will probably consume about 27-GPH. With 140-gallons useable fuel, this give an endurance at full-throttle of about five hours.

These endurance factors can be scaled proportionally for a lower volume of fuel capacity, and you can evaluate the effect of reducing fuel capacity based on your own expectations. Clearly the RANGE of the boat will be reduced in direct proportion to the fuel capacity reduction, so a reduction in fuel capacity by a factor of 0.66 will similarly reduce the boat's range by that same factor.

Regarding the possibility of improved fuel economy and better boat speed by reducing fuel tank capacity, I don't really see much room there. Gasoline weights about 6.25-lbs per gallon. Let us say you reduced fuel tank capacity to 100-gallons, a reduction of about 50-gallons. The weight saving would then be about 315-lbs.

This reduction in weight of 315-lbs must be judged for a benefit by comparison to the total boat weight. The dry hull weight of a 23 CONQUEST is 2,900-lbs. A rough estimate of the weight of the boat underway with fuel, gear, engines, and passengers will be about 6,000-lbs. Now we can assess the potential for improved top speed based on a weight reduction by a ratio of 5685/6000 or a ratio of 0.947. We know that as a general rule boat speed is proportion to the power to weight ratio to the 0.5 exponent. We can then estimate the speed increase as likely to improve by a factor if about 1.02-times. For example, if the boat could reach 40-MPH, then with a weight reduction of 315-lbs the boat speed might improve to 40.8-MPH.

A speed increase of less than 1-MPH is not a significant change in the boat's top speed. Unless you were participating in some sort of speed competition event with the boat, I doubt anyone would even notice a change in top speed of less than 1-MPH.

Any advantage in lower fuel weight only occurs when the new, reduced-capacity tank is full and is compared to the prior larger-capacity tank when it would be full. Note that you can realize the same improvement in reduced fuel weight just by not filling a larger capacity tank to full all the time.

Regarding improved fuel economy, the estimate of that can be derived in the same way. For a particular boat speed, the power needed to maintain that boat speed will decrease if the weight decreases. However, as we saw in the speed calculation, the difference is very small, and the fuel savings will be similarly small, on the order of less than five-percent at most.

In summary, the advice you received that you could improve performance and improve fuel economy by reducing the weight of the fuel carried is quite true. All reductions in weight on a boat result in improvements in speed or improvements in fuel economy. But the magnitude of the benefit of reducing the fuel tank capacity, even cutting it by a third to 100-gallons from 150-gallons, seems quite insignificant. The reason for the minimal effect is that weight saved is a very small fraction of the total weight of the boat when underway.

On the basis I have laid out above, I do not see there is a particular mandate to reduce fuel tank capacity in order to obtain a very small improvement in top speed or in fuel economy. A much greater effect of reduced fuel tank capacity will be a very significant and linear reduction in the boat's fuel endurance and thus the boat's range. Whether a benefit of a marginal improvement in speed or fuel economy is worth a significant reduction in range will be a matter of individual judgement.

[jimh]

[Should the replacement tank be] made with aluminum or plastic polyethylene?

Replacement fuel tanks made from aluminum can be fabricated by cutting and welding aluminum sheet. There are many fabricators who can cut and weld a replacement aluminum fuel tank to fit the particular shape of a boat's fuel tank cavity.

Replacement fuel tanks made from polyethylene are generally made by using custom molds. Most polyethylene fuel tanks are only available in pre-molded sizes, and fitment of an off-the-shelf pre-molded polyethylene fuel tank to the particular shape of the fuel tank cavity on a 23 CONQUEST boat would depend on finding an appropriate model made by some tank fabricator. I doubt it would be cost effective to have a custom mold made to fabricate an exact fit tank to the boat.

From what material was the original fuel tank made?

[SeaTrek2]

Thanks Jim for the very detailed explanation, very informative.